I would here willingly have proceeded to exhibit the whole chain of
truths which I deduced from these primary but as with a view to this it
would have been necessary now to treat of many questions in dispute
among the earned, with whom I do not wish to be embroiled, I believe
that it will be better for me to refrain from this exposition, and only
mention in general what these truths are, that the more judicious may
be able to determine whether a more special account of them would
conduce to the public advantage. I have ever remained firm in my
original resolution to suppose no other principle than that of which I
have recently availed myself in demonstrating the existence of God and
of the soul, and to accept as true nothing that did not appear to me
more clear and certain than the demonstrations of the geometers had
formerly appeared; and yet I venture to state that not only have I
found means to satisfy myself in a short time on all the principal
difficulties which are usually treated of in philosophy, but I have
also observed certain laws established in nature by God in such a
manner, and of which he has impressed on our minds such notions, that
after we have reflected sufficiently upon these, we cannot doubt that
they are accurately observed in all that exists or takes place in the
world and farther, by considering the concatenation of these laws, it
appears to me that I have discovered many truths more useful and more
important than all I had before learned, or even had expected to learn.
But because I have essayed to expound the chief of these discoveries in
a treatise which certain considerations prevent me from publishing, I
cannot make the results known more conveniently than by here giving a
summary of the contents of this treatise. It was my design to comprise
in it all that, before I set myself to write it, I thought I knew of
the nature of material objects. But like the painters who, finding
themselves unable to represent equally well on a plain surface all the
different faces of a solid body, select one of the chief, on which
alone they make the light fall, and throwing the rest into the shade,
allow them to appear only in so far as they can be seen while looking
at the principal one; so, fearing lest I should not be able to compense
in my discourse all that was in my mind, I resolved to expound singly,
though at considerable length, my opinions regarding light; then to
take the opportunity of adding something on the sun and the fixed
stars, since light almost wholly proceeds from them; on the heavens
since they transmit it; on the planets, comets, and earth, since they
reflect it; and particularly on all the bodies that are upon the earth,
since they are either colored, or transparent, or luminous; and finally
on man, since he is the spectator of these objects. Further, to enable
me to cast this variety of subjects somewhat into the shade, and to
express my judgment regarding them with greater freedom, without being
necessitated to adopt or refute the opinions of the learned, I resolved
to leave all the people here to their disputes, and to speak only of
what would happen in a new world, if God were now to create somewhere
in the imaginary spaces matter sufficient to compose one, and were to
agitate variously and confusedly the different parts of this matter, so
that there resulted a chaos as disordered as the poets ever feigned,
and after that did nothing more than lend his ordinary concurrence to
nature, and allow her to act in accordance with the laws which he had
established. On this supposition, I, in the first place, described this
matter, and essayed to represent it in such a manner that to my mind
there can be nothing clearer and more intelligible, except what has
been recently said regarding God and the soul; for I even expressly
supposed that it possessed none of those forms or qualities which are
so debated in the schools, nor in general anything the knowledge of
which is not so natural to our minds that no one can so much as imagine
himself ignorant of it. Besides, I have pointed out what are the laws
of nature; and, with no other principle upon which to found my
reasonings except the infinite perfection of God, I endeavored to
demonstrate all those about which there could be any room for doubt,
and to prove that they are such, that even if God had created more
worlds, there could have been none in which these laws were not
observed. Thereafter, I showed how the greatest part of the matter of
this chaos must, in accordance with these laws, dispose and arrange
itself in such a way as to present the appearance of heavens; how in
the meantime some of its parts must compose an earth and some planets
and comets, and others a sun and fixed stars. And, making a digression
at this stage on the subject of light, I expounded at considerable
length what the nature of that light must be which is found in the sun
and the stars, and how thence in an instant of time it traverses the
immense spaces of the heavens, and how from the planets and comets it
is reflected towards the earth. To this I likewise added much
respecting the substance, the situation, the motions, and all the
different qualities of these heavens and stars; so that I thought I had
said enough respecting them to show that there is nothing observable in
the heavens or stars of our system that must not, or at least may not
appear precisely alike in those of the system which I described. I came
next to speak of the earth in particular, and to show how, even though
I had expressly supposed that God had given no weight to the matter of
which it is composed, this should not prevent all its parts from
tending exactly to its center; how with water and air on its surface,
the disposition of the heavens and heavenly bodies, more especially of
the moon, must cause a flow and ebb, like in all its circumstances to
that observed in our seas, as also a certain current both of water and
air from east to west, such as is likewise observed between the
tropics; how the mountains, seas, fountains, and rivers might naturally
be formed in it, and the metals produced in the mines, and the plants
grow in the fields and in general, how all the bodies which are
commonly denominated mixed or composite might be generated and, among
other things in the discoveries alluded to inasmuch as besides the
stars, I knew nothing except fire which produces light, I spared no
pains to set forth all that pertains to its nature,--the manner of its
production and support, and to explain how heat is sometimes found
without light, and light without heat; to show how it can induce
various colors upon different bodies and other diverse qualities; how
it reduces some to a liquid state and hardens others; how it can
consume almost all bodies, or convert them into ashes and smoke; and
finally, how from these ashes, by the mere intensity of its action, it
forms glass: for as this transmutation of ashes into glass appeared to
me as wonderful as any other in nature, I took a special pleasure in
describing it. I was not, however, disposed, from these circumstances,
to conclude that this world had been created in the manner I described;
for it is much more likely that God made it at the first such as it was
to be. But this is certain, and an opinion commonly received among
theologians, that the action by which he now sustains it is the same
with that by which he originally created it; so that even although he
had from the beginning given it no other form than that of chaos,
provided only he had established certain laws of nature, and had lent
it his concurrence to enable it to act as it is wont to do, it may be
believed, without discredit to the miracle of creation, that, in this
way alone, things purely material might, in course of time, have become
such as we observe them at present; and their nature is much more
easily conceived when they are beheld coming in this manner gradually
into existence, than when they are only considered as produced at once
in a finished and perfect state.
From the description of inanimate bodies and plants, I passed to
animals, and particularly to man. But since I had not as yet sufficient
knowledge to enable me to treat of these in the same manner as of the
rest, that is to say, by deducing effects from their causes, and by
showing from what elements and in what manner nature must produce them,
I remained satisfied with the supposition that God formed the body of
man wholly like to one of ours, as well in the external shape of the
members as in the internal conformation of the organs, of the same
matter with that I had described, and at first placed in it no rational
soul, nor any other principle, in room of the vegetative or sensitive
soul, beyond kindling in the heart one of those fires without light,
such as I had already described, and which I thought was not different
from the heat in hay that has been heaped together before it is dry, or
that which causes fermentation in new wines before they are run clear
of the fruit. For, when I examined the kind of functions which might,
as consequences of this supposition, exist in this body, I found
precisely all those which may exist in us independently of all power of
thinking, and consequently without being in any measure owing to the
soul; in other words, to that part of us which is distinct from the
body, and of which it has been said above that the nature distinctively
consists in thinking, functions in which the animals void of reason may
be said wholly to resemble us; but among which I could not discover any
of those that, as dependent on thought alone, belong to us as men,
while, on the other hand, I did afterwards discover these as soon as I
supposed God to have created a rational soul, and to have annexed it to
this body in a particular manner which I described.
But, in order to show how I there handled this matter, I mean here to
give the explication of the motion of the heart and arteries, which, as
the first and most general motion observed in animals, will afford the
means of readily determining what should be thought of all the rest.
And that there may be less difficulty in understanding what I am about
to say on this subject, I advise those who are not versed in anatomy,
before they commence the perusal of these observations, to take the
trouble of getting dissected in their presence the heart of some large
animal possessed of lungs (for this is throughout sufficiently like the
human), and to have shown to them its two ventricles or cavities: in
the first place, that in the right side, with which correspond two very
ample tubes, viz., the hollow vein (_vena cava_), which is the
principal receptacle of the blood, and the trunk of the tree, as it
were, of which all the other veins in the body are branches; and the
arterial vein (_vena arteriosa_), inappropriately so denominated, since
it is in truth only an artery, which, taking its rise in the heart, is
divided, after passing out from it, into many branches which presently
disperse themselves all over the lungs; in the second place, the cavity
in the left side, with which correspond in the same manner two canals
in size equal to or larger than the preceding, viz., the venous artery
(_arteria venosa_), likewise inappropriately thus designated, because
it is simply a vein which comes from the lungs, where it is divided
into many branches, interlaced with those of the arterial vein, and
those of the tube called the windpipe, through which the air we breathe
enters; and the great artery which, issuing from the heart, sends its
branches all over the body. I should wish also that such persons were
carefully shown the eleven pellicles which, like so many small valves,
open and shut the four orifices that are in these two cavities, viz.,
three at the entrance of the hollow veins where they are disposed in
such a manner as by no means to prevent the blood which it contains
from flowing into the right ventricle of the heart, and yet exactly to
prevent its flowing out; three at the entrance to the arterial vein,
which, arranged in a manner exactly the opposite of the former, readily
permit the blood contained in this cavity to pass into the lungs, but
hinder that contained in the lungs from returning to this cavity; and,
in like manner, two others at the mouth of the venous artery, which
allow the blood from the lungs to flow into the left cavity of the
heart, but preclude its return; and three at the mouth of the great
artery, which suffer the blood to flow from the heart, but prevent its
reflux. Nor do we need to seek any other reason for the number of these
pellicles beyond this that the orifice of the venous artery being of an
oval shape from the nature of its situation, can be adequately closed
with two, whereas the others being round are more conveniently closed
with three. Besides, I wish such persons to observe that the grand
artery and the arterial vein are of much harder and firmer texture than
the venous artery and the hollow vein; and that the two last expand
before entering the heart, and there form, as it were, two pouches
denominated the auricles of the heart, which are composed of a
substance similar to that of the heart itself; and that there is always
more warmth in the heart than in any other part of the body--and
finally, that this heat is capable of causing any drop of blood that
passes into the cavities rapidly to expand and dilate, just as all
liquors do when allowed to fall drop by drop into a highly heated
vessel.
For, after these things, it is not necessary for me to say anything
more with a view to explain the motion of the heart, except that when
its cavities are not full of blood, into these the blood of necessity
flows,--from the hollow vein into the right, and from the venous artery
into the left; because these two vessels are always full of blood, and
their orifices, which are turned towards the heart, cannot then be
closed. But as soon as two drops of blood have thus passed, one into
each of the cavities, these drops which cannot but be very large,
because the orifices through which they pass are wide, and the vessels
from which they come full of blood, are immediately rarefied, and
dilated by the heat they meet with. In this way they cause the whole
heart to expand, and at the same time press home and shut the five
small valves that are at the entrances of the two vessels from which
they flow, and thus prevent any more blood from coming down into the
heart, and becoming more and more rarefied, they push open the six
small valves that are in the orifices of the other two vessels, through
which they pass out, causing in this way all the branches of the
arterial vein and of the grand artery to expand almost simultaneously
with the heart which immediately thereafter begins to contract, as do
also the arteries, because the blood that has entered them has cooled,
and the six small valves close, and the five of the hollow vein and of
the venous artery open anew and allow a passage to other two drops of
blood, which cause the heart and the arteries again to expand as
before. And, because the blood which thus enters into the heart passes
through these two pouches called auricles, it thence happens that their
motion is the contrary of that of the heart, and that when it expands
they contract. But lest those who are ignorant of the force of
mathematical demonstrations and who are not accustomed to distinguish
true reasons from mere verisimilitudes, should venture, without
examination, to deny what has been said, I wish it to be considered
that the motion which I have now explained follows as necessarily from
the very arrangement of the parts, which may be observed in the heart
by the eye alone, and from the heat which may be felt with the fingers,
and from the nature of the blood as learned from experience, as does
the motion of a clock from the power, the situation, and shape of its
counterweights and wheels.
But if it be asked how it happens that the blood in the veins, flowing
in this way continually into the heart, is not exhausted, and why the
arteries do not become too full, since all the blood which passes
through the heart flows into them, I need only mention in reply what
has been written by a physician of England, who has the honor of having
broken the ice on this subject, and of having been the first to teach
that there are many small passages at the extremities of the arteries,
through which the blood received by them from the heart passes into the
small branches of the veins, whence it again returns to the heart; so
that its course amounts precisely to a perpetual circulation. Of this
we have abundant proof in the ordinary experience of surgeons, who, by
binding the arm with a tie of moderate straitness above the part where
they open the vein, cause the blood to flow more copiously than it
would have done without any ligature; whereas quite the contrary would
happen were they to bind it below; that is, between the hand and the
opening, or were to make the ligature above the opening very tight. For
it is manifest that the tie, moderately straightened, while adequate to
hinder the blood already in the arm from returning towards the heart by
the veins, cannot on that account prevent new blood from coming forward
through the arteries, because these are situated below the veins, and
their coverings, from their greater consistency, are more difficult to
compress; and also that the blood which comes from the heart tends to
pass through them to the hand with greater force than it does to return
from the hand to the heart through the veins. And since the latter
current escapes from the arm by the opening made in one of the veins,
there must of necessity be certain passages below the ligature, that
is, towards the extremities of the arm through which it can come
thither from the arteries. This physician likewise abundantly
establishes what he has advanced respecting the motion of the blood,
from the existence of certain pellicles, so disposed in various places
along the course of the veins, in the manner of small valves, as not to
permit the blood to pass from the middle of the body towards the
extremities, but only to return from the extremities to the heart; and
farther, from experience which shows that all the blood which is in the
body may flow out of it in a very short time through a single artery
that has been cut, even although this had been closely tied in the
immediate neighborhood of the heart and cut between the heart and the
ligature, so as to prevent the supposition that the blood flowing out
of it could come from any other quarter than the heart.
But there are many other circumstances which evince that what I have
alleged is the true cause of the motion of the blood: thus, in the
first place, the difference that is observed between the blood which
flows from the veins, and that from the arteries, can only arise from
this, that being rarefied, and, as it were, distilled by passing
through the heart, it is thinner, and more vivid, and warmer
immediately after leaving the heart, in other words, when in the
arteries, than it was a short time before passing into either, in other
words, when it was in the veins; and if attention be given, it will be
found that this difference is very marked only in the neighborhood of
the heart; and is not so evident in parts more remote from it. In the
next place, the consistency of the coats of which the arterial vein and
the great artery are composed, sufficiently shows that the blood is
impelled against them with more force than against the veins. And why
should the left cavity of the heart and the great artery be wider and
larger than the right cavity and the arterial vein, were it not that
the blood of the venous artery, having only been in the lungs after it
has passed through the heart, is thinner, and rarefies more readily,
and in a higher degree, than the blood which proceeds immediately from
the hollow vein? And what can physicians conjecture from feeling the
pulse unless they know that according as the blood changes its nature
it can be rarefied by the warmth of the heart, in a higher or lower
degree, and more or less quickly than before? And if it be inquired how
this heat is communicated to the other members, must it not be admitted
that this is effected by means of the blood, which, passing through the
heart, is there heated anew, and thence diffused over all the body?
Whence it happens, that if the blood be withdrawn from any part, the
heat is likewise withdrawn by the same means; and although the heart
were as-hot as glowing iron, it would not be capable of warming the
feet and hands as at present, unless it continually sent thither new
blood. We likewise perceive from this, that the true use of respiration
is to bring sufficient fresh air into the lungs, to cause the blood
which flows into them from the right ventricle of the heart, where it
has been rarefied and, as it were, changed into vapors, to become
thick, and to convert it anew into blood, before it flows into the left
cavity, without which process it would be unfit for the nourishment of
the fire that is there. This receives confirmation from the
circumstance, that it is observed of animals destitute of lungs that
they have also but one cavity in the heart, and that in children who
cannot use them while in the womb, there is a hole through which the
blood flows from the hollow vein into the left cavity of the heart, and
a tube through which it passes from the arterial vein into the grand
artery without passing through the lung. In the next place, how could
digestion be carried on in the stomach unless the heart communicated
heat to it through the arteries, and along with this certain of the
more fluid parts of the blood, which assist in the dissolution of the
food that has been taken in? Is not also the operation which converts
the juice of food into blood easily comprehended, when it is considered
that it is distilled by passing and repassing through the heart perhaps
more than one or two hundred times in a day? And what more need be
adduced to explain nutrition, and the production of the different
humors of the body, beyond saying, that the force with which the blood,
in being rarefied, passes from the heart towards the extremities of the
arteries, causes certain of its parts to remain in the members at which
they arrive, and there occupy the place of some others expelled by
them; and that according to the situation, shape, or smallness of the
pores with which they meet, some rather than others flow into certain
parts, in the same way that some sieves are observed to act, which, by
being variously perforated, serve to separate different species of
grain? And, in the last place, what above all is here worthy of
observation, is the generation of the animal spirits, which are like a
very subtle wind, or rather a very pure and vivid flame which,
continually ascending in great abundance from the heart to the brain,
thence penetrates through the nerves into the muscles, and gives motion
to all the members; so that to account for other parts of the blood
which, as most agitated and penetrating, are the fittest to compose
these spirits, proceeding towards the brain, it is not necessary to
suppose any other cause, than simply, that the arteries which carry
them thither proceed from the heart in the most direct lines, and that,
according to the rules of mechanics which are the same with those of
nature, when many objects tend at once to the same point where there is
not sufficient room for all (as is the case with the parts of the blood
which flow forth from the left cavity of the heart and tend towards the
brain), the weaker and less agitated parts must necessarily be driven
aside from that point by the stronger which alone in this way reach it
I had expounded all these matters with sufficient minuteness in the
treatise which I formerly thought of publishing. And after these, I had
shown what must be the fabric of the nerves and muscles of the human
body to give the animal spirits contained in it the power to move the
members, as when we see heads shortly after they have been struck off
still move and bite the earth, although no longer animated; what
changes must take place in the brain to produce waking, sleep, and
dreams; how light, sounds, odors, tastes, heat, and all the other
qualities of external objects impress it with different ideas by means
of the senses; how hunger, thirst, and the other internal affections
can likewise impress upon it divers ideas; what must be understood by
the common sense (_sensus communis_) in which these ideas are received,
by the memory which retains them, by the fantasy which can change them
in various ways, and out of them compose new ideas, and which, by the
same means, distributing the animal spirits through the muscles, can
cause the members of such a body to move in as many different ways, and
in a manner as suited, whether to the objects that are presented to its
senses or to its internal affections, as can take place in our own case
apart from the guidance of the will. Nor will this appear at all
strange to those who are acquainted with the variety of movements
performed by the different automata, or moving machines fabricated by
human industry, and that with help of but few pieces compared with the
great multitude of bones, muscles, nerves, arteries, veins, and other
parts that are found in the body of each animal. Such persons will look
upon this body as a machine made by the hands of God, which is
incomparably better arranged, and adequate to movements more admirable
than is any machine of human invention. And here I specially stayed to
show that, were there such machines exactly resembling organs and
outward form an ape or any other irrational animal, we could have no
means of knowing that they were in any respect of a different nature
from these animals; but if there were machines bearing the image of our
bodies, and capable of imitating our actions as far as it is morally
possible, there would still remain two most certain tests whereby to
know that they were not therefore really men. Of these the first is
that they could never use words or other signs arranged in such a
manner as is competent to us in order to declare our thoughts to
others: for we may easily conceive a machine to be so constructed that
it emits vocables, and even that it emits some correspondent to the
action upon it of external objects which cause a change in its organs;
for example, if touched in a particular place it may demand what we
wish to say to it; if in another it may cry out that it is hurt, and
such like; but not that it should arrange them variously so as
appositely to reply to what is said in its presence, as men of the
lowest grade of intellect can do. The second test is, that although
such machines might execute many things with equal or perhaps greater
perfection than any of us, they would, without doubt, fail in certain
others from which it could be discovered that they did not act from
knowledge, but solely from the disposition of their organs: for while
reason is an universal instrument that is alike available on every
occasion, these organs, on the contrary, need a particular arrangement
for each particular action; whence it must be morally impossible that
there should exist in any machine a diversity of organs sufficient to
enable it to act in all the occurrences of life, in the way in which
our reason enables us to act. Again, by means of these two tests we may
likewise know the difference between men and brutes. For it is highly
deserving of remark, that there are no men so dull and stupid, not even
idiots, as to be incapable of joining together different words, and
thereby constructing a declaration by which to make their thoughts
understood; and that on the other hand, there is no other animal,
however perfect or happily circumstanced, which can do the like. Nor
does this inability arise from want of organs: for we observe that
magpies and parrots can utter words like ourselves, and are yet unable
to speak as we do, that is, so as to show that they understand what
they say; in place of which men born deaf and dumb, and thus not less,
but rather more than the brutes, destitute of the organs which others
use in speaking, are in the habit of spontaneously inventing certain
signs by which they discover their thoughts to those who, being usually
in their company, have leisure to learn their language. And this proves
not only that the brutes have less reason than man, but that they have
none at all: for we see that very little is required to enable a person
to speak; and since a certain inequality of capacity is observable
among animals of the same species, as well as among men, and since some
are more capable of being instructed than others, it is incredible that
the most perfect ape or parrot of its species, should not in this be
equal to the most stupid infant of its kind or at least to one that was
crack-brained, unless the soul of brutes were of a nature wholly
different from ours. And we ought not to confound speech with the
natural movements which indicate the passions, and can be imitated by
machines as well as manifested by animals; nor must it be thought with
certain of the ancients, that the brutes speak, although we do not
understand their language. For if such were the case, since they are
endowed with many organs analogous to ours, they could as easily
communicate their thoughts to us as to their fellows. It is also very
worthy of remark, that, though there are many animals which manifest
more industry than we in certain of their actions, the same animals are
yet observed to show none at all in many others: so that the
circumstance that they do better than we does not prove that they are
endowed with mind, for it would thence follow that they possessed
greater reason than any of us, and could surpass us in all things; on
the contrary, it rather proves that they are destitute of reason, and
that it is nature which acts in them according to the disposition of
their organs: thus it is seen, that a clock composed only of wheels and
weights can number the hours and measure time more exactly than we with
all our skin.
I had after this described the reasonable soul, and shown that it could
by no means be educed from the power of matter, as the other things of
which I had spoken, but that it must be expressly created; and that it
is not sufficient that it be lodged in the human body exactly like a
pilot in a ship, unless perhaps to move its members, but that it is
necessary for it to be joined and united more closely to the body, in
order to have sensations and appetites similar to ours, and thus
constitute a real man. I here entered, in conclusion, upon the subject
of the soul at considerable length, because it is of the greatest
moment: for after the error of those who deny the existence of God, an
error which I think I have already sufficiently refuted, there is none
that is more powerful in leading feeble minds astray from the straight
path of virtue than the supposition that the soul of the brutes is of
the same nature with our own; and consequently that after this life we
have nothing to hope for or fear, more than flies and ants; in place of
which, when we know how far they differ we much better comprehend the
reasons which establish that the soul is of a nature wholly independent
of the body, and that consequently it is not liable to die with the
latter and, finally, because no other causes are observed capable of
destroying it, we are naturally led thence to judge that it is
immortal.