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The Principles of Philosophy by Rene Descartes

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things revealed, we ought to assent to nothing that we do not
clearly apprehend.

Above all, we must impress on our memory the infallible rule, that
what God has revealed is incomparably more certain than anything
else; and that, we ought to submit our belief to the Divine
authority rather than to our own judgment, even although perhaps the
light of reason should, with the greatest clearness and evidence,
appear to suggest to us something contrary to what is revealed. But
in things regarding which there is no revelation, it is by no means
consistent with the character of a philosopher to accept as true
what he has not ascertained to be such, and to trust more to the
senses, in other words, to the inconsiderate judgments of childhood
than to the dictates of mature reason.



I. The grounds on which the existence of material things may be
known with certainty.

Although we are all sufficiently persuaded of the existence of
material things, yet, since this was before called in question by
us, and since we reckoned the persuasion of their existence as among
the prejudices of our childhood, it is now necessary for us to
investigate the grounds on which this truth may be known with
certainty. In the first place, then, it cannot be doubted that every
perception we have comes to us from some object different from our
mind; for it is not in our power to cause ourselves to experience
one perception rather than another, the perception being entirely
dependent on the object which affects our senses. It may, indeed, be
matter of inquiry whether that object be God, or something different
from God; but because we perceive, or rather, stimulated by sense,
clearly and distinctly apprehend, certain matter extended in length,
breadth, and thickness, the various parts of which have different
figures and motions, and give rise to the sensation we have of
colours, smells, pain, etc., God would, without question, deserve to
be regarded as a deceiver, if he directly and of himself presented
to our mind the idea of this extended matter, or merely caused it to
be presented to us by some object which possessed neither extension,
figure, nor motion. For we clearly conceive this matter as entirely
distinct from God, and from ourselves, or our mind; and appear even
clearly to discern that the idea of it is formed in us on occasion
of objects existing out of our minds, to which it is in every
respect similar. But since God cannot deceive us, for this is
repugnant to his nature, as has been already remarked, we must
unhesitatingly conclude that there exists a certain object extended
in length, breadth, and thickness, and possessing all those
properties which we clearly apprehend to belong to what is extended.
And this extended substance is what we call body or matter.

II. How we likewise know that the human body is closely connected
with the mind.

We ought also to conclude that a certain body is more closely united
to our mind than any other, because we clearly observe that pain and
other sensations affect us without our foreseeing them; and these,
the mind is conscious, do not arise from itself alone, nor pertain
to it, in so far as it is a thing which thinks, but only in so far
as it is united to another thing extended and movable, which is
called the human body. But this is not the place to treat in detail
of this matter.

III. That the perceptions of the senses do not teach us what is in
reality in things, but what is beneficial of hurtful to the
composite whole of mind and body.

It will be sufficient to remark that the perceptions of the senses
are merely to be referred to this intimate union of the human body
and mind, and that they usually make us aware of what, in external
objects, may be useful or adverse to this union, but do not present
to us these objects as they are in themselves, unless occasionally
and by accident. For, after this observation, we will without
difficulty lay aside the prejudices of the senses, and will have
recourse to our understanding alone on this question by reflecting
carefully on the ideas implanted in it by nature.

IV. That the nature of body consists not in weight hardness, colour
and the like, but in extension alone.

In this way we will discern that the nature of matter or body,
considered in general, does not consist in its being hard, or
ponderous, or coloured, or that which affects our senses in any
other way, but simply in its being a substance extended in length,
breadth, and depth. For with respect to hardness, we know nothing of
it by sense farther than that the parts of hard bodies resist the
motion of our hands on coming into contact with them; but if every
time our hands moved towards any part, all the bodies in that place
receded as quickly as our hands approached, we should never feel
hardness; and yet we have no reason to believe that bodies which
might thus recede would on this account lose that which makes them
bodies. The nature of body does not, therefore, consist in hardness.
In the same way, it may be shown that weight, colour, and all the
other qualities of this sort, which are perceived in corporeal
matter, may be taken from it, itself meanwhile remaining entire: it
thus follows that the nature of body depends on none of these.

V. That the truth regarding the nature of body is obscured by the
opinions respecting rarefaction and a vacuum with which we are pre-

There still remain two causes to prevent its being fully admitted
that the true nature of body consists in extension alone. The first
is the prevalent opinion, that most bodies admit of being so
rarefied and condensed that, when rarefied, they have greater
extension than when condensed; and some even have subtilized to such
a degree as to make a distinction between the substance of body and
its quantity, and between quantity itself and extension. The second
cause is this, that where we conceive only extension in length,
breadth, and depth, we are not in the habit of saying that body is
there, but only space and further void space, which the generality
believe to be a mere negation.

VI. In what way rarefaction takes place.

But with regard to rarefaction and condensation, whoever gives his
attention to his own thoughts, and admits nothing of which he is not
clearly conscious, will not suppose that there is anything in those
processes further than a change of figure in the body rarefied or
condensed: so that, in other words, rare bodies are those between
the parts of which there are numerous distances filled with other
bodies; and dense bodies, on the other hand, those whose parts
approaching each other, either diminish these distances or take them
wholly away, in the latter of which cases the body is rendered
absolutely dense. The body, however, when condensed, has not,
therefore, less extension than when the parts embrace a greater
space, owing to their removal from each other, and their dispersion
into branches. For we ought not to attribute to it the extension of
the pores or distances which its parts do not occupy when it is
rarefied, but to the other bodies that fill these interstices; just
as when we see a sponge full of water or any other liquid, we do not
suppose that each part of the sponge has on this account greater
extension than when compressed and dry, but only that its pores are
wider, and therefore that the body is diffused over a larger space.

VII. That rarefaction cannot be intelligibly explained unless in
the way here proposed.

And indeed I am unable to discover the force of the reasons which
have induced some to say that rarefaction is the result of the
augmentation of the quantity of body, rather than to explain it on
the principle exemplified in the case of a sponge. For although when
air or water is rarefied we do not see any of the pores that are
rendered large, or the new body that is added to occupy them, it is
yet less agreeable to reason to suppose something that is
unintelligible for the purpose of giving a verbal and merely
apparent explanation of the rarefaction of bodies, than to conclude,
because of their rarefaction, that there are pores or distances
between the parts which are increased in size, and filled with some
new body. Nor ought we to refrain from assenting to this
explanation, because we perceive this new body by none of our
senses, for there is no reason which obliges us to believe that we
should perceive by our senses all the bodies in existence. And we
see that it is very easy to explain rarefaction in this manner, but
impossible in any other; for, in fine, there would be, as appears to
me, a manifest contradiction in supposing that any body was
increased by a quantity or extension which it had not before,
without the addition to it of a new extended substance, in other
words, of another body, because it is impossible to conceive any
addition of extension or quantity to a thing without supposing the
addition of a substance having quantity or extension, as will more
clearly appear from what follows.

VIII. That quantity and number differ only in thought (RATIONE)
from that which has quantity and is numbered.

For quantity differs from extended substance, and number from what
is numbered, not in reality but merely in our thought; so that, for
example, we may consider the whole nature of a corporeal substance
which is comprised in a space of ten feet, although we do not attend
to this measure of ten feet, for the obvious reason that the thing
conceived is of the same nature in any part of that space as in the
whole; and, on the other hand, we can conceive the number ten, as
also a continuous quantity of ten feet, without thinking of this
determinate substance, because the concept of the number ten is
manifestly the same whether we consider a number of ten feet or ten
of anything else; and we can conceive a continuous quantity of ten
feet without thinking of this or that determinate substance,
although we cannot conceive it without some extended substance of
which it is the quantity. It is in reality, however, impossible that
any, even the least part, of such quantity or extension, can be
taken away, without the retrenchment at the same time of as much of
the substance, nor, on the other hand, can we lessen the substance,
without at the same time taking as much from the quantity or

IX. That corporeal substance, when distinguished from its quantity,
is confusedly conceived as something incorporeal.

Although perhaps some express themselves otherwise on this matter, I
am nevertheless convinced that they do not think differently from
what I have now said: for when they distinguish (corporeal)
substance from extension or quantity, they either mean nothing by
the word (corporeal) substance, or they form in their minds merely a
confused idea of incorporeal substance, which they falsely attribute
to corporeal, and leave to extension the true idea of this corporeal
substance; which extension they call an accident, but with such
impropriety as to make it easy to discover that their words are not
in harmony with their thoughts.

X. What space or internal place is.

Space or internal place, and the corporeal substance which is
comprised in it, are not different in reality, but merely in the
mode in which they are wont to be conceived by us. For, in truth,
the same extension in length, breadth, and depth, which constitutes
space, constitutes body; and the difference between them lies only
in this, that in body we consider extension as particular, and
conceive it to change with the body; whereas in space we attribute
to extension a generic unity, so that after taking from a certain
space the body which occupied it, we do not suppose that we have at
the same time removed the extension of the space, because it appears
to us that the same extension remains there so long as it is of the
same magnitude and figure, and preserves the same situation in
respect to certain bodies around it, by means of which we determine
this space.

XI. How space is not in reality different from corporeal substance.

And indeed it will be easy to discern that it is the same extension
which constitutes the nature of body as of space, and that these two
things are mutually diverse only as the nature of the genus and
species differs from that of the individual, provided we reflect on
the idea we have of any body, taking a stone for example, and reject
all that is not essential to the nature of body. In the first place,
then, hardness may be rejected, because if the stone were liquefied
or reduced to powder, it would no longer possess hardness, and yet
would not cease to be a body; colour also may be thrown out of
account, because we have frequently seen stones so transparent as to
have no colour; again, we may reject weight, because we have the
case of fire, which, though very light, is still a body; and,
finally, we may reject cold, heat, and all the other qualities of
this sort, either because they are not considered as in the stone,
or because, with the change of these qualities, the stone is not
supposed to have lost the nature of body. After this examination we
will find that nothing remains in the idea of body, except that it
is something extended in length, breadth, and depth; and this
something is comprised in our idea of space, not only of that which
is full of body, but even of what is called void space.

XII. How space differs from body in our mode of conceiving it.

There is, however, some difference between them in the mode of
conception; for if we remove a stone from the space or place in
which it was, we conceive that its extension also is taken away,
because we regard this as particular, and inseparable from the stone
itself: but meanwhile we suppose that the same extension of place in
which this stone was remains, although the place of the stone be
occupied by wood, water, air, or by any other body, or be even
supposed vacant, because we now consider extension in general, and
think that the same is common to stones, wood, water, air, and other
bodies, and even to a vacuum itself, if there is any such thing,
provided it be of the same magnitude and figure as before, and
preserve the same situation among the external bodies which
determine this space.

XIII. What external place is.

The reason of which is, that the words place and space signify
nothing really different from body which is said to be in place, but
merely designate its magnitude, figure, and situation among other
bodies. For it is necessary, in order to determine this situation,
to regard certain other bodies which we consider as immovable; and,
according as we look to different bodies, we may see that the same
thing at the same time does and does not change place. For example,
when a vessel is being carried out to sea, a person sitting at the
stern may be said to remain always in one place, if we look to the
parts of the vessel, since with respect to these he preserves the
same situation; and on the other hand, if regard be had to the
neighbouring shores, the same person will seem to be perpetually
changing place, seeing he is constantly receding from one shore and
approaching another. And besides, if we suppose that the earth
moves, and that it makes precisely as much way from west to east as
the vessel from east to west, we will again say that the person at
the stern does not change his place, because this place will be
determined by certain immovable points which we imagine to be in the
heavens. But if at length we are persuaded that there are no points
really immovable in the universe, as will hereafter be shown to be
probable, we will thence conclude that nothing has a permanent place
unless in so far as it is fixed by our thought.

XIV. Wherein place and space differ.

The terms place and space, however, differ in signification, because
place more expressly designates situation than magnitude or figure,
while, on the other hand, we think of the latter when we speak of
space. For we frequently say that a thing succeeds to the place of
another, although it be not exactly of the same magnitude or figure;
but we do not therefore admit that it occupies the same space as the
other; and when the situation is changed we say that the place also
is changed, although there are the same magnitude and figure as
before: so that when we say that a thing is in a particular place,
we mean merely that it is situated in a determinate way in respect
of certain other objects; and when we add that it occupies such a
space or place, we understand besides that it is of such determinate
magnitude and figure as exactly to fill this space.

XV. How external place is rightly taken for the superficies of the
surrounding body.

And thus we never indeed distinguish space from extension in length,
breadth, and depth; we sometimes, however, consider place as in the
thing placed, and at other times as out of it. Internal place indeed
differs in no way from space; but external place may be taken for
the superficies that immediately surrounds the thing placed. It
ought to be remarked that by superficies we do not here understand
any part of the surrounding body, but only the boundary between the
surrounding and surrounded bodies, which is nothing more than a
mode; or at least that we speak of superficies in general which is
no part of one body rather than another, but is always considered
the same, provided it retain the same magnitude and figure. For
although the whole surrounding body with its superficies were
changed, it would not be supposed that the body which was surrounded
by it had therefore changed its place, if it meanwhile preserved the
same situation with respect to the other bodies that are regarded as
immovable. Thus, if we suppose that a boat is carried in one
direction by the current of a stream, and impelled by the wind in
the opposite with an equal force, so that its situation with respect
to the banks is not changed, we will readily admit that it remains
in the same place, although the whole superficies which surrounds it
is incessantly changing.

XVI. That a vacuum or space in which there is absolutely no body is
repugnant to reason.

With regard to a vacuum, in the philosophical sense of the term,
that is, a space in which there is no substance, it is evident that
such does not exist, seeing the extension of space or internal place
is not different from that of body. For since from this alone, that
a body has extension in length, breadth, and depth, we have reason
to conclude that it is a substance, it being absolutely
contradictory that nothing should possess extension, we ought to
form a similar inference regarding the space which is supposed void,
viz., that since there is extension in it there is necessarily also

XVII. That a vacuum in the ordinary use of the term does not
exclude all body.

And, in truth, by the term vacuum in its common use, we do not mean
a place or space in which there is absolutely nothing, but only a
place in which there is none of those things we presume ought to be
there. Thus, because a pitcher is made to hold water, it is said to
be empty when it is merely filled with air; or if there are no fish
in a fish-pond, we say there is nothing in it, although it be full
of water; thus a vessel is said to be empty, when, in place of the
merchandise which it was designed to carry, it is loaded with sand
only, to enable it to resist the violence of the wind; and, finally,
it is in the same sense that we say space is void when it contains
nothing sensible, although it contain created and self-subsisting
matter; for we are not in the habit of considering the bodies near
us, unless in so far as they cause in our organs of sense,
impressions strong enough to enable us to perceive them. And if, in
place of keeping in mind what ought to be understood by these terms
a vacuum and nothing, we afterwards suppose that in the space we
called a vacuum, there is not only no sensible object, but no object
at all, we will fall into the same error as if, because a pitcher in
which there is nothing but air, is, in common speech, said to be
empty, we were therefore to judge that the air contained in it is
not a substance (RES SUBSISTENS).

XVIII. How the prejudice of an absolute vacuum is to be corrected.

We have almost all fallen into this error from the earliest age,
for, observing that there is no necessary connection between a
vessel and the body it contains, we thought that God at least could
take from a vessel the body which occupied it, without it being
necessary that any other should be put in the place of the one
removed. But that we may be able now to correct this false opinion,
it is necessary to remark that there is in truth no connection
between the vessel and the particular body which it contains, but
that there is an absolutely necessary connection between the concave
figure of the vessel and the extension considered generally which
must be comprised in this cavity; so that it is not more
contradictory to conceive a mountain without a valley than such a
cavity without the extension it contains, or this extension apart
from an extended substance, for, as we have often said, of nothing
there can be no extension. And accordingly, if it be asked what
would happen were God to remove from a vessel all the body contained
in it, without permitting another body to occupy its place, the
answer must be that the sides of the vessel would thus come into
proximity with each other. For two bodies must touch each other when
there is nothing between them, and it is manifestly contradictory
for two bodies to be apart, in other words, that there should be a
distance between them, and this distance yet be nothing; for all
distance is a mode of extension, and cannot therefore exist without
an extended substance.

XIX. That this confirms what was said of rarefaction.

After we have thus remarked that the nature of corporeal substance
consists only in its being an extended thing, and that its extension
is not different from that which we attribute to space, however
empty, it is easy to discover the impossibility of any one of its
parts in any way whatsoever occupying more space at one time than at
another, and thus of being otherwise rarefied than in the way
explained above; and it is easy to perceive also that there cannot
be more matter or body in a vessel when it is filled with lead or
gold, or any other body however heavy and hard, than when it but
contains air and is supposed to be empty: for the quantity of the
parts of which a body is composed does not depend on their weight or
hardness, but only on the extension, which is always equal in the
same vase.

XX. That from this the non-existence of atoms may likewise be

We likewise discover that there cannot exist any atoms or parts of
matter that are of their own nature indivisible. For however small
we suppose these parts to be, yet because they are necessarily
extended, we are always able in thought to divide any one of them
into two or more smaller parts, and may accordingly admit their
divisibility. For there is nothing we can divide in thought which we
do not thereby recognize to be divisible; and, therefore, were we to
judge it indivisible our judgment would not be in harmony with the
knowledge we have of the thing; and although we should even suppose
that God had reduced any particle of matter to a smallness so
extreme that it did not admit of being further divided, it would
nevertheless be improperly styled indivisible, for though God had
rendered the particle so small that it was not in the power of any
creature to divide it, he could not however deprive himself of the
ability to do so, since it is absolutely impossible for him to
lessen his own omnipotence, as was before observed. Wherefore,
absolutely speaking, the smallest extended particle is always
divisible, since it is such of its very nature.

XXI. It is thus also demonstrated that the extension of the world
is indefinite.

We further discover that this world or the whole (universitas) of
corporeal substance, is extended without limit, for wherever we fix
a limit, we still not only imagine beyond it spaces indefinitely
extended, but perceive these to be truly imaginable, in other words,
to be in reality such as we imagine them; so that they contain in
them corporeal substance indefinitely extended, for, as has been
already shown at length, the idea of extension which we conceive in
any space whatever is plainly identical with the idea of corporeal

XXII. It also follows that the matter of the heavens and earth is
the same, and that there cannot be a plurality of worlds.

And it may also be easily inferred from all this that the earth and
heavens are made of the same matter; and that even although there
were an infinity of worlds, they would all be composed of this
matter; from which it follows that a plurality of worlds is
impossible, because we clearly conceive that the matter whose nature
consists only in its being an extended substance, already wholly
occupies all the imaginable spaces where these other worlds could
alone be, and we cannot find in ourselves the idea of any other

XXIII. That all the variety of matter, or the diversity of its
forms, depends on motion.

There is therefore but one kind of matter in the whole universe, and
this we know only by its being extended. All the properties we
distinctly perceive to belong to it are reducible to its capacity of
being divided and moved according to its parts; and accordingly it
is capable of all those affections which we perceive can arise from
the motion of its parts. For the partition of matter in thought
makes no change in it; but all variation of it, or diversity of
form, depends on motion. The philosophers even seem universally to
have observed this, for they said that nature was the principle of
motion and rest, and by nature they understood that by which all
corporeal things become such as they are found in experience.

XXIV. What motion is, taking the term in its common use.

But motion (viz., local, for I can conceive no other kind of motion,
and therefore I do not think we ought to suppose there is any other
in nature), in the ordinary sense of the term, is nothing more than
the action by which a body passes from one place to another. And
just as we have remarked above that the same thing may be said to
change and not to change place at the same time, so also we may say
that the same thing is at the same time moved and not moved. Thus,
for example, a person seated in a vessel which is setting sail,
thinks he is in motion if he look to the shore that he has left, and
consider it as fixed; but not if he regard the ship itself, among
the parts of which he preserves always the same situation. Moreover,
because we are accustomed to suppose that there is no motion without
action, and that in rest there is the cessation of action, the
person thus seated is more properly said to be at rest than in
motion, seeing he is not conscious of being in action.

XXV. What motion is properly so called.

But if, instead of occupying ourselves with that which has no
foundation, unless in ordinary usage, we desire to know what ought
to be understood by motion according to the truth of the thing, we
may say, in order to give it a determinate nature, that it is THE
REGARD AS AT REST, to the vicinity of other bodies. By a body as a
part of matter, I understand all that which is transferred together,
although it be perhaps composed of several parts, which in
themselves have other motions; and I say that it is the transporting
and not the force or action which transports, with the view of
showing that motion is always in the movable thing, not in that
which moves; for it seems to me that we are not accustomed to
distinguish these two things with sufficient accuracy. Farther, I
understand that it is a mode of the movable thing, and not a
substance, just as figure is a property of the thing figured, and
repose of that which is at rest.



I. That we cannot think too highly of the works of God.

Having now ascertained certain principles of material things, which
were sought, not by the prejudices of the senses, but by the light
of reason, and which thus possess so great evidence that we cannot
doubt of their truth, it remains for us to consider whether from
these alone we can deduce the explication of all the phenomena of
nature. We will commence with those phenomena that are of the
greatest generality, and upon which the others depend, as, for
example, with the general structure of this whole visible world. But
in order to our philosophizing aright regarding this, two things are
first of all to be observed. The first is, that we should ever bear
in mind the infinity of the power and goodness of God, that we may
not fear falling into error by imagining his works to be too great,
beautiful, and perfect, but that we may, on the contrary, take care
lest, by supposing limits to them of which we have no certain
knowledge, we appear to think less highly than we ought of the power
of God.

II. That we ought to beware lest, in our presumption, we imagine
that the ends which God proposed to himself in the creation of the
world are understood by us.

The second is, that we should beware of presuming too highly of
ourselves, as it seems we should do if we supposed certain limits to
the world, without being assured of their existence either by
natural reasons or by divine revelation, as if the power of our
thought extended beyond what God has in reality made; but likewise
still more if we persuaded ourselves that all things were created by
God for us only, or if we merely supposed that we could comprehend
by the power of our intellect the ends which God proposed to himself
in creating the universe.

III. In what sense it may be said that all things were created for
the sake of man.

For although, as far as regards morals, it may be a pious thought to
believe that God made all things for us, seeing we may thus be
incited to greater gratitude and love toward him; and although it is
even in some sense true, because there is no created thing of which
we cannot make some use, if it be only that of exercising our mind
in considering it, and honouring God on account of it, it is yet by
no means probable that all things were created for us in this way
that God had no other end in their creation; and this supposition
would be plainly ridiculous and inept in physical reasoning, for we
do not doubt but that many things exist, or formerly existed and
have now ceased to be, which were never seen or known by man, and
were never of use to him.



CLXXXVIII. Of what is to be borrowed from disquisitions on animals
and man to advance the knowledge of material objects.

I should add nothing farther to this the Fourth Part of the
Principles of Philosophy, did I purpose carrying out my original
design of writing a Fifth and Sixth Part, the one treating of things
possessed of life, that is, animals and plants, and the other of
man. But because I have not yet acquired sufficient knowledge of all
the matters of which I should desire to treat in these two last
parts, and do not know whether I shall ever have sufficient leisure
to finish them, I will here subjoin a few things regarding the
objects of our senses, that I may not, for the sake of the latter,
delay too long the publication of the former parts, or of what may
be desiderated in them, which I might have reserved for explanation
in those others: for I have hitherto described this earth, and
generally the whole visible world, as if it were merely a machine in
which there was nothing at all to consider except the figures and
motions of its parts, whereas our senses present to us many other
things, for example colours, smells, sounds, and the like, of which,
if I did not speak at all, it would be thought I had omitted the
explication of the majority of the objects that are in nature.

CLXXXIX. What perception (SENSUS) is, and how we perceive.

We must know, therefore, that although the human soul is united to
the whole body, it has, nevertheless, its principal seat in the
brain, where alone it not only understands and imagines, but also
perceives; and this by the medium of the nerves, which are extended
like threads from the brain to all the other members, with which
they are so connected that we can hardly touch any one of them
without moving the extremities of some of the nerves spread over it;
and this motion passes to the other extremities of those nerves
which are collected in the brain round the seat of the soul,
[Footnote: *** FOOTNOTE NOT VISIBLE IN PAGE IMAGE (#98, Text p 195)]
as I have already explained with sufficient minuteness in the fourth
chapter of the Dioptrics. But the movements which are thus excited
in the brain by the nerves variously affect the soul or mind, which
is intimately conjoined with the brain, according to the diversity
of the motions themselves. And the diverse affections of the mind or
thoughts that immediately arise from these motions, are called
perceptions of the senses (SENSUUM PERCEPTIONES), or, as we commonly
speak, sensations (SENSUS).

CXC. Of the distinction of the senses; and, first, of the internal,
that is, of the affections of the mind (passions), and the natural

The varieties of these sensations depend, firstly, on the diversity
of the nerves themselves, and, secondly, of the movements that are
made in each nerve. We have not, however, as many different senses
as there are nerves. We can distinguish but seven principal classes
of nerves, of which two belong to the internal, and the other five
to the external senses. The nerves which extend to the stomach, the
oesophagus, the fauces, and the other internal parts that are
subservient to our natural wants, constitute one of our internal
senses. This is called the natural appetite (APPETITUS NATURALIS).
The other internal sense, which embraces all the emotions
(COMMOTIONES) of the mind or passions, and affections, as joy,
sadness, love, hate, and the like, depends upon the nerves which
extend to the heart and the parts about the heart, and are
exceedingly small; for, by way of example, when the blood happens to
be pure and well tempered, so that it dilates in the heart more
readily and strongly than usual, this so enlarges and moves the
small nerves scattered around the orifices, that there is thence a
corresponding movement in the brain, which affects the mind with a
certain natural feeling of joy; and as often as these same nerves
are moved in the same way, although this is by other causes, they
excite in our mind the same feeling (sensus, sentiment). Thus, the
imagination of the enjoyment of a good does not contain in itself
the feeling of joy, but it causes the animal spirits to pass from
the brain to the muscles in which these nerves are inserted; and
thus dilating the orifices of the heart, it also causes these small
nerves to move in the way appointed by nature to afford the
sensation of joy. Thus, when we receive news, the mind first of all
judges of it, and if the news be good, it rejoices with that
intellectual joy (GAUDIUM INTELLECTUALE) which is independent of any
emotion (COMMOTIO) of the body, and which the Stoics did not deny to
their wise man [although they supposed him exempt from all passion].
But as soon as this joy passes from the understanding to the
imagination, the spirits flow from the brain to the muscles that are
about the heart, and there excite the motion of the small nerves, by
means of which another motion is caused in the brain, which affects
the mind with the sensation of animal joy (LAETITIA ANIMALIS). On
the same principle, when the blood is so thick that it flows but
sparingly into the ventricles of the heart, and is not there
sufficiently dilated, it excites in the same nerves a motion quite
different from the preceding, which, communicated to the brain,
gives to the mind the sensation of sadness, although the mind itself
is perhaps ignorant of the cause of its sadness. And all the other
causes which move these nerves in the same way may also give to the
mind the same sensation. But the other movements of the same nerves
produce other effects, as the feelings of love, hate, fear, anger,
etc., as far as they are merely affections or passions of the mind;
in other words, as far as they are confused thoughts which the mind
has not from itself alone, but from its being closely joined to the
body, from which it receives impressions; for there is the widest
difference between these passions and the distinct thoughts which we
have of what ought to be loved, or chosen, or shunned, etc.,
[although these are often enough found together]. The natural
appetites, as hunger, thirst, and the others, are likewise
sensations excited in the mind by means of the nerves of the
stomach, fauces, and other parts, and are entirely different from
the will which we have to eat, drink, [and to do all that which we
think proper for the conservation of our body]; but, because this
will or appetition almost always accompanies them, they are
therefore named appetites.

CXCI. Of the external senses; and first of touch.

We commonly reckon the external senses five in number, because there
are as many different kinds of objects which move the nerves and
their organs, and an equal number of kinds of confused thoughts
excited in the soul by these emotions. In the first place, the
nerves terminating in the skin of the whole body can be touched
through this medium by any terrene objects whatever, and moved by
these wholes, in one way by their hardness, in another by their
gravity, in a third by their heat, in a fourth by their humidity,
etc.--and in as many diverse modes as they are either moved or
hindered from their ordinary motion, to that extent are diverse
sensations excited in the mind, from which a corresponding number of
tactile qualities derive their appellations. Besides this, when
these nerves are moved a little more powerfully than usual, but not
nevertheless to the degree by which our body is in any way hurt,
there thus arises a sensation of titillation, which is naturally
agreeable to the mind, because it testifies to it of the powers of
the body with which it is joined, [in that the latter can suffer the
action causing this titillation, without being hurt]. But if this
action be strong enough to hurt our body in any way, this gives to
our mind the sensation of pain. And we thus see why corporeal
pleasure and pain, although sensations of quite an opposite
character, arise nevertheless from causes nearly alike.

CXCII. Of taste.

In the second place, the other nerves scattered over the tongue and
the parts in its vicinity are diversely moved by the particles of
the same bodies, separated from each other and floating in the
saliva in the mouth, and thus cause sensations of diverse tastes
according to the diversity of figure in these particles. [Footnote:
In the French this section begins, "Taste, after touch the grossest
of the senses," etc.]

CXCIII. Of smell.

Thirdly, two nerves also or appendages of the brain, for they do not
go beyond the limits of the skull, are moved by the particles of
terrestrial bodies, separated and flying in the air, not indeed by
all particles indifferently, but by those only that are sufficiently
subtle and penetrating to enter the pores of the bone we call the
spongy, when drawn into the nostrils, and thus to reach the nerves.
From the different motions of these particles arise the sensations
of the different smells.

CXCIV. Of hearing.

Fourthly, there are two nerves within the ears, so attached to three
small bones that are mutually sustaining, and the first of which
rests on the small membrane that covers the cavity we call the
tympanum of the ear, that all the diverse vibrations which the
surrounding air communicates to this membrane are transmitted to the
mind by these nerves, and these vibrations give rise, according to
their diversity, to the sensations of the different sounds.

CXCV. Of sight.

Finally, the extremities of the optic nerves, composing the coat in
the eyes called the retina, are not moved by the air nor by any
terrestrial object, but only by the globules of the second element,
whence we have the sense of light and colours: as I have already at
sufficient length explained in the Dioptrics and treatise of
Meteors. [Footnote: In the French this section begins, "Finally,
sight is the most subtle of all the senses," etc.]

CXCVI. That the soul perceives only in so far as it is in the

It is clearly established, however, that the soul does not perceive
in so far as it is in each member of the body, but only in so far as
it is in the brain, where the nerves by their movements convey to it
the diverse actions of the external objects that touch the parts of
the body in which they are inserted. For, in the first place, there
are various maladies, which, though they affect the brain alone, yet
bring disorder upon, or deprive us altogether of the use of, our
senses, just as sleep, which affects the brain only, and yet takes
from us daily during a great part of our time the faculty of
perception, which afterwards in our waking state is restored to us.
The second proof is, that though there be no disease in the brain,
[or in the members in which the organs of the external senses are],
it is nevertheless sufficient to take away sensation from the part
of the body where the nerves terminate, if only the movement of one
of the nerves that extend from the brain to these members be
obstructed in any part of the distance that is between the two. And
the last proof is, that we sometimes feel pain as if in certain of
our members, the cause of which, however, is not in these members
where it is felt, but somewhere nearer the brain, through which the
nerves pass that give to the mind the sensation of it. I could
establish this fact by innumerable experiments; I will here,
however, merely refer to one of them. A girl suffering from a bad
ulcer in the hand, had her eyes bandaged whenever the surgeon came
to visit her, not being able to bear the sight of the dressing of
the sore; and, the gangrene having spread, after the expiry of a few
days the arm was amputated from the elbow [without the girl's
knowledge]; linen cloths tied one above the other were substituted
in place of the part amputated, so that she remained for some time
without knowing that the operation had been performed, and meanwhile
she complained of feeling various pains, sometimes in one finger of
the hand that was cut off, and sometimes in another. The only
explanation of this is, that the nerves which before stretched
downwards from the brain to the hand, and then terminated in the arm
close to the elbow, were there moved in the same way as they
required to be moved before in the hand for the purpose of
impressing on the mind residing in the brain the sensation of pain
in this or that finger. [And this clearly shows that the pain of the
hand is not felt by the mind in so far as it is in the hand, but in
so far as it is in the brain.]

CXCVII. That the nature of the mind is such that from the motion
alone of body the various sensations can be excited in it.

In the next place, it can be proved that our mind is of such a
nature that the motions of the body alone are sufficient to excite
in it all sorts of thoughts, without it being necessary that these
should in any way resemble the motions which give rise to them, and
especially that these motions can excite in it those confused
thoughts called sensations (SENSUS, SENSATIONES). For we see that
words, whether uttered by the voice or merely written, excite in our
minds all kinds of thoughts and emotions. On the same paper, with
the same pen and ink, by merely moving the point of the pen over the
paper in a particular way, we can trace letters that will raise in
the minds of our readers the thoughts of combats, tempests, or the
furies, and the passions of indignation and sorrow; in place of
which, if the pen be moved in another way hardly different from the
former, this slight change will cause thoughts widely different from
the above, such as those of repose, peace, pleasantness, and the
quite opposite passions of love and joy. Some one will perhaps
object that writing and speech do not immediately excite in the mind
any passions, or imaginations of things different from the letters
and sounds, but afford simply the knowledge of these, on occasion of
which the mind, understanding the signification of the words,
afterwards excites in itself the imaginations and passions that
correspond to the words. But what will be said of the sensations of
pain and titillation? The motion merely of a sword cutting a part of
our skin causes pain, [but does not on that account make us aware of
the motion or figure of the sword]. And it is certain that this
sensation of pain is not less different from the motion that causes
it, or from that of the part of our body which the sword cuts, than
are the sensations we have of colour, sound, odour, or taste. On
this ground we may conclude that our mind is of such a nature that
the motions alone of certain bodies can also easily excite in it all
the other sensations, as the motion of a sword excites in it the
sensation of pain.

CXCVIII. That by our senses we know nothing of external objects
beyond their figure [or situation], magnitude, and motion.

Besides, we observe no such difference between the nerves as to lead
us to judge that one set of them convey to the brain from the organs
of the external senses anything different from another, or that
anything at all reaches the brain besides the local motion of the
nerves themselves. And we see that local motion alone causes in us
not only the sensation of titillation and of pain, but also of light
and sounds. For if we receive a blow on the eye of sufficient force
to cause the vibration of the stroke to reach the retina, we see
numerous sparks of fire, which, nevertheless, are not out of our
eye; and when we stop our ear with our finger, we hear a humming
sound, the cause of which can only proceed from the agitation of the
air that is shut up within it. Finally, we frequently observe that
heat [hardness, weight], and the other sensible qualities, as far as
they are in objects, and also the forms of those bodies that are
purely material, as, for example, the forms of fire, are produced in
them by the motion of certain other bodies, and that these in their
turn likewise produce other motions in other bodies. And we can
easily conceive how the motion of one body may be caused by that of
another, and diversified by the size, figure, and situation of its
parts, but we are wholly unable to conceive how these same things
(viz., size, figure, and motion), can produce something else of a
nature entirely different from themselves, as, for example, those
substantial forms and real qualities which many philosophers suppose
to be in bodies; nor likewise can we conceive how these qualities or
forms possess force to cause motions in other bodies. But since we
know, from the nature of our soul, that the diverse motions of body
are sufficient to produce in it all the sensations which it has, and
since we learn from experience that several of its sensations are in
reality caused by such motions, while we do not discover that
anything besides these motions ever passes from the organs of the
external senses to the brain, we have reason to conclude that we in
no way likewise apprehend that in external objects, which we call
light, colour, smell, taste, sound, heat or cold, and the other
tactile qualities, or that which we call their substantial forms,
unless as the various dispositions of these objects which have the
power of moving our nerves in various ways. [Footnote: "the diverse
figures, situations, magnitudes, and motions of their parts."--

CXCIX. That there is no phenomenon of nature whose explanation has
been omitted in this treatise.

And thus it may be gathered, from an enumeration that is easily
made, that there is no phenomenon of nature whose explanation has
been omitted in this treatise; for beyond what is perceived by the
senses, there is nothing that can be considered a phenomenon of
nature. But leaving out of account motion, magnitude, figure, [and
the situation of the parts of each body], which I have explained as
they exist in body, we perceive nothing out of us by our senses
except light, colours, smells, tastes, sounds, and the tactile
qualities; and these I have recently shown to be nothing more, at
least so far as they are known to us, than certain dispositions of
the objects, consisting in magnitude, figure, and motion.

CC. That this treatise contains no principles which are not
universally received; and that this philosophy is not new, but of
all others the most ancient and common.

But I am desirous also that it should be observed that, though I
have here endeavoured to give an explanation of the whole nature of
material things, I have nevertheless made use of no principle which
was not received and approved by Aristotle, and by the other
philosophers of all ages; so that this philosophy, so far from being
new, is of all others the most ancient and common: for I have in
truth merely considered the figure, motion, and magnitude of bodies,
and examined what must follow from their mutual concourse on the
principles of mechanics, which are confirmed by certain and daily
experience. But no one ever doubted that bodies are moved, and that
they are of various sizes and figures, according to the diversity of
which their motions also vary, and that from mutual collision those
somewhat greater than others are divided into many smaller, and thus
change figure. We have experience of the truth of this, not merely
by a single sense, but by several, as touch, sight, and hearing: we
also distinctly imagine and understand it. This cannot be said of
any of the other things that fall under our senses, as colours,
sounds, and the like; for each of these affects but one of our
senses, and merely impresses upon our imagination a confused image
of itself, affording our understanding no distinct knowledge of what
it is.

CCI. That sensible bodies are composed of insensible particles.

But I allow many particles in each body that are perceived by none
of our senses, and this will not perhaps be approved of by those who
take the senses for the measure of the knowable. [We greatly wrong
human reason, however, as appears to me, if we suppose that it does
not go beyond the eye-sight]; for no one can doubt that there are
bodies so small as not to be perceptible by any of our senses,
provided he only consider what is each moment added to those bodies
that are being increased little by little, and what is taken from
those that are diminished in the same way. A tree increases daily,
and it is impossible to conceive how it becomes greater than it was
before, unless we at the same time conceive that some body is added
to it. But who ever observed by the senses those small bodies that
are in one day added to a tree while growing? Among the philosophers
at least, those who hold that quantity is indefinitely divisible,
ought to admit that in the division the parts may become so small as
to be wholly imperceptible. And indeed it ought not to be a matter
of surprise, that we are unable to perceive very minute bodies; for
the nerves that must be moved by objects to cause perception are not
themselves very minute, but are like small cords, being composed of
a quantity of smaller fibres, and thus the most minute bodies are
not capable of moving them. Nor do I think that any one who makes
use of his reason will deny that we philosophize with much greater
truth when we judge of what takes place in those small bodies which
are imperceptible from their minuteness only, after the analogy of
what we see occurring in those we do perceive, [and in this way
explain all that is in nature, as I have essayed to do in this
treatise], than when we give an explanation of the same things by
inventing I know not what novelties, that have no relation to the
things we actually perceive, [as first matter, substantial forms,
and all that grand array of qualities which many are in the habit of
supposing, each of which is more difficult to comprehend than all
that is professed to be explained by means of them].

CCII. That the philosophy of Democritus is not less different from
ours than from the common. [Footnote: "that of Aristotle or the

But it may be said that Democritus also supposed certain corpuscles
that were of various figures, sizes, and motions, from the heaping
together and mutual concourse of which all sensible bodies arose;
and, nevertheless, his mode of philosophizing is commonly rejected
by all. To this I reply that the philosophy of Democritus was never
rejected by any one, because he allowed the existence of bodies
smaller than those we perceive, and attributed to them diverse
sizes, figures, and motions, for no one can doubt that there are in
reality such, as we have already shown; but it was rejected, in the
first place, because he supposed that these corpuscles were
indivisible, on which ground I also reject it; in the second place,
because he imagined there was a vacuum about them, which I show to
be impossible; thirdly, because he attributed gravity to these
bodies, of which I deny the existence in any body, in so far as a
body is considered by itself, because it is a quality that depends
on the relations of situation and motion which several bodies bear
to each other; and, finally, because he has not explained in
particular how all things arose from the concourse of corpuscles
alone, or, if he gave this explanation with regard to a few of them,
his whole reasoning was far from being coherent, [or such as would
warrant us in extending the same explanation to the whole of
nature]. This, at least, is the verdict we must give regarding his
philosophy, if we may judge of his opinions from what has been
handed down to us in writing. I leave it to others to determine
whether the philosophy I profess possesses a valid coherency, [and
whether on its principles we can make the requisite number of
deductions; and, inasmuch as the consideration of figure, magnitude,
and motion has been admitted by Aristotle and by all the others, as
well as by Democritus, and since I reject all that the latter has
supposed, with this single exception, while I reject generally all
that has been supposed by the others, it is plain that this mode of
philosophizing has no more affinity with that of Democritus than of
any other particular sect].

CCIII. How we may arrive at the knowledge of the figures,
[magnitudes], and motions of the insensible particles of bodies.

But, since I assign determinate figures, magnitudes, and motions to
the insensible particles of bodies, as if I had seen them, whereas I
admit that they do not fall under the senses, some one will perhaps
demand how I have come by my knowledge of them. [To this I reply,
that I first considered in general all the clear and distinct
notions of material things that are to be found in our
understanding, and that, finding no others except those of figures,
magnitudes, and motions, and of the rules according to which these
three things can be diversified by each other, which rules are the
principles of geometry and mechanics, I judged that all the
knowledge man can have of nature must of necessity be drawn from
this source; because all the other notions we have of sensible
things, as confused and obscure, can be of no avail in affording us
the knowledge of anything out of ourselves, but must serve rather to
impede it]. Thereupon, taking as my ground of inference the simplest
and best known of the principles that have been implanted in our
minds by nature, I considered the chief differences that could
possibly subsist between the magnitudes, and figures, and situations
of bodies insensible on account of their smallness alone, and what
sensible effects could be produced by their various modes of coming
into contact; and afterwards, when I found like effects in the
bodies that we perceive by our senses, I judged that they could have
been thus produced, especially since no other mode of explaining
them could be devised. And in this matter the example of several
bodies made by art was of great service to me: for I recognize no
difference between these and natural bodies beyond this, that the
effects of machines depend for the most part on the agency of
certain instruments, which, as they must bear some proportion to the
hands of those who make them, are always so large that their figures
and motions can be seen; in place of which, the effects of natural
bodies almost always depend upon certain organs so minute as to
escape our senses. And it is certain that all the rules of mechanics
belong also to physics, of which it is a part or species, [so that
all that is artificial is withal natural]: for it is not less
natural for a clock, made of the requisite number of wheels, to mark
the hours, than for a tree, which has sprung from this or that seed,
to produce the fruit peculiar to it. Accordingly, just as those who
are familiar with automata, when they are informed of the use of a
machine, and see some of its parts, easily infer from these the way
in which the others, that are not seen by them, are made; so from
considering the sensible effects and parts of natural bodies, I have
essayed to determine the character of their causes and insensible

CCIV. That, touching the things which our senses do not perceive,
it is sufficient to explain how they can be, [and that this is all
that Aristotle has essayed].

But here some one will perhaps reply, that although I have supposed
causes which could produce all natural objects, we ought not on this
account to conclude that they were produced by these causes; for,
just as the same artisan can make two clocks, which, though they
both equally well indicate the time, and are not different in
outward appearance, have nevertheless nothing resembling in the
composition of their wheels; so doubtless the Supreme Maker of
things has an infinity of diverse means at his disposal, by each of
which he could have made all the things of this world to appear as
we see them, without it being possible for the human mind to know
which of all these means he chose to employ. I most freely concede
this; and I believe that I have done all that was required, if the
causes I have assigned are such that their effects accurately
correspond to all the phenomena of nature, without determining
whether it is by these or by others that they are actually produced.
And it will be sufficient for the use of life to know the causes
thus imagined, for medicine, mechanics, and in general all the arts
to which the knowledge of physics is of service, have for their end
only those effects that are sensible, and that are accordingly to be
reckoned among the phenomena of nature. [Footnote: "have for their
end only to apply certain sensible bodies to each other in such a
way that, in the course of natural causes, certain sensible effects
may be produced; and we will be able to accomplish this quite as
well by considering the series of certain causes thus imagined,
although false, as if they were the true, since this series is
supposed similar as far as regards sensible effects."-French.]

And lest it should be supposed that Aristotle did, or professed to
do, anything more than this, it ought to be remembered that he
himself expressly says, at the commencement of the seventh chapter
of the first book of the Meteorologies, that, with regard to things
which are not manifest to the senses, he thinks to adduce sufficient
reasons and demonstrations of them, if he only shows that they may
be such as he explains them. [Footnote: words in Greek]

CCV. That nevertheless there is a moral certainty that all the
things of this world are such as has been here shown they may be.

But nevertheless, that I may not wrong the truth by supposing it
less certain than it is, I will here distinguish two kinds of
certitude. The first is called moral, that is, a certainty
sufficient for the conduct of life, though, if we look to the
absolute power of God, what is morally certain may be false. [Thus,
those who never visited Rome do not doubt that it is a city of
Italy, though it might be that all from whom they got their
information were deceived]. Again, if any one, wishing to decipher a
letter written in Latin characters that are not placed in regular
order, bethinks himself of reading a B wherever an A is found, and a
C wherever there is a B, and thus of substituting in place of each
letter the one which follows it in the order of the alphabet, and if
by this means he finds that there are certain Latin words composed
of these, he will not doubt that the true meaning of the writing is
contained in these words, although he may discover this only by
conjecture, and although it is possible that the writer of it did
not arrange the letters on this principle of alphabetical order, but
on some other, and thus concealed another meaning in it: for this is
so improbable [especially when the cipher contains a number of
words] as to seem incredible. But they who observe how many things
regarding the magnet, fire, and the fabric of the whole world, are
here deduced from a very small number of principles, though they
deemed that I had taken them up at random and without grounds, will
yet perhaps acknowledge that it could hardly happen that so many
things should cohere if these principles were false.

CCVI. That we possess even more than a moral certainty of it.

Besides, there are some, even among natural, things which we judge
to be absolutely certain. [Absolute certainty arises when we judge
that it is impossible a thing can be otherwise than as we think it].
This certainty is founded on the metaphysical ground, that, as God
is supremely good and the source of all truth, the faculty of
distinguishing truth from error which he gave us, cannot be
fallacious so long as we use it aright, and distinctly perceive
anything by it. Of this character are the demonstrations of
mathematics, the knowledge that material things exist, and the clear
reasonings that are formed regarding them. The results I have given
in this treatise will perhaps be admitted to a place in the class of
truths that are absolutely certain, if it be considered that they
are deduced in a continuous series from the first and most
elementary principles of human knowledge; especially if it be
sufficiently understood that we can perceive no external objects
unless some local motion be caused by them in our nerves, and that
such motion cannot be caused by the fixed stars, owing to their
great distance from us, unless a motion be also produced in them and
in the whole heavens lying between them and us: for these points
being admitted, all the others, at least the more general doctrines
which I have advanced regarding the world or earth [e. g., the
fluidity of the heavens, Part III., Section XLVI.], will appear to
be almost the only possible explanations of the phenomena they

CCVII. That, however, I submit all my opinions to the authority of
the church.

Nevertheless, lest I should presume too far, I affirm nothing, but
submit all these my opinions to the authority of the church and the
judgment of the more sage; and I desire no one to believe anything I
may have said, unless he is constrained to admit it by the force and
evidence of reason.

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