Department of History
University of California, Irvine
 Instructor:    Dr. Barbara J. Becker

Week 5.  The Age of Enlightenment.

excerpts from
The World, or Treatise on Light (1629-1633)
by René Descartes (1596-1650)
Translated by Michael S. Mahoney

...there are as many different heavens as there are stars, and, since the number of stars is indefinite, so too is the number of heavens.

On the Number of Elements and on Their Qualities

I conceive of the first, which one may call the element of fire, as the most subtle and penetrating fluid there is in the world....  Thus, there is never a passage so narrow, nor an angle so small, among the parts of other bodies, where the parts of this element do not penetrate without any difficulty and which they do not fill exactly.

As for the second, which one may take to be the element of air, I conceive of it also as a very subtle fluid in comparison with the third; but in comparison with the first there is need to attribute some size and shape to each of its parts and to imagine them as just about all round and joined together like grains of sand or dust.  Thus, they cannot arrange themselves so well, nor so press against one another that there do not always remain around them many small intervals into which it is much easier for the first element to slide than for the parts of the second to change shape expressly in order to fill them....

Beyond these two elements, I accept only a third, to wit, that of earth.  Its parts I judge to be as much larger and to move as much less swiftly in comparison with those of the second as those of the second in comparison with those of the first.  Indeed, I believe it is enough to conceive of it as one or more large masses, of which the parts have very little or no motion that might cause them to change position with respect to one another....

[I]f we consider in general all the bodies of which the universe is composed, we will find among them only three sorts that can be called large and be counted among the principal parts, to wit, the sun and the fixed stars as the first sort, the heavens as the second, and the earth with the planets and the comets as the third.  That is why we have good reason to think that the sun and the fixed stars have no other form than that of the wholly pure first element, the heavens that of the second, and the earth with the planets and comets that of the third....

Description of a New World, and on the Qualities of the Matter of Which it is Composed

For a short time, then, allow your thought to wander beyond this world to view another, wholly new one, which I shall cause to unfold before it in imaginary spaces.  The philosophers tell us that these spaces are infinite, and they should very well be believed, since it is they themselves who have made the spaces so.  Yet, in order that this infinity not impede us and not embarrass us, let us not try to go all the way to the end; let us enter in only so far that we can lose from view all the creatures that God made five or six thousand years ago and, after having stopped there in some fixed place, let us suppose that God creates from anew so much matter all about us that, in whatever direction our imagination can extend itself, it no longer perceives any place that is empty....

[E]ven though our imagination seems to be able to extend itself to infinity, and this new matter is not assumed to be infinite, we can nonetheless well suppose that it fills spaces much greater than all those we shall have imagined....  [L]et us not permit our imagination to extend itself as far as it could, but let us purposely restrict it to a determinate space that is no greater, say, than the distance between the earth and the principal stars of the firmament, and let us suppose that the matter that God shall have created extends quite far beyond in all directions, out to an indefinite distance....

[M]y plan is not to set out (as they do) the things that are in fact in the true world, but only to make up as I please from [this matter] a [world] in which there is nothing that the densest minds are not capable of conceiving, and which nevertheless could be created exactly the way I have made it up....

On the Laws of Nature of this New World

...I will set out here two or three of the principal rules according to which one must think God to cause the nature of this new world to act and which will suffice, I believe, for you to know all the others.

The first is that each individual part of matter always continues to remain in the same state unless collision with others constrains it to change that state.  That is to say, if the part has some size, it will never become smaller unless others divide it; if it is round or square, it will never change that shape without others forcing it to do so; if it is stopped in some place, it will never depart from that place unless others chase it away; and if it has once begun to move, it will always continue with an equal force until others stop or retard it....

I suppose as a second rule that, when one of these bodies pushes another, it cannot give the other any motion except by losing as much of its own at the same time; nor can it take away from the other body's motion unless its own is increased by as much....

I will add as a third rule that, when a body is moving, even if its motion most often takes place along a curved line and ... can never take place along any line that is not in some way circular, nevertheless each of its individual parts tends always to continue its motion along a straight line....

I could set out here many additional rules for determining in detail when and how and by how much the motion of each body can be diverted and increased or decreased by colliding with others, something that comprises summarily all the effects of nature.  But I shall be content with showing you that, besides the three laws that I have explained, I wish to suppose no others but those that most certainly follow from the eternal truths on which the mathematicians are wont to support their most certain and most evident demonstrations; the truths, I say, according to which God Himself has taught us He disposed all things in number, weight, and measure.

The knowledge of those laws is so natural to our souls that we cannot but judge them infallible when we conceive them distinctly, nor doubt that, if God had created many worlds, the laws would be as true in all of them as in this one....

Nonetheless, in consequence of this, I do not promise you to set out here exact demonstrations of all the things I will say.  It will be enough for me to open to you the path by which you will be able to find them yourselves, whenever you take the trouble to look for them.  Most minds lose interest when one makes things too easy for them.  And to compose here a setting that pleases you, I must employ shadow as well as bright colors.  Thus I will be content to pursue the description I have begun, as if having no other design than to tell you a fable.

On the Formation of the Sun and the Stars of the New World

Whatever inequality and confusion we might suppose God put among the parts of matter at the beginning, the parts must, according to the laws He imposed on nature, thereafter almost all have been reduced to one size and to one middling motion and thus have taken the form of the second element as I described it above.  For to consider this matter in the state in which it could have been before God began to move it, one should imagine it as the hardest and most solid body in the world.  And, since one could not push any part of such a body without pushing or pulling all the other parts by the same means, so one must imagine that the action or the force of moving or dividing, which had first been placed in some of the parts of matter, spread out and distributed itself in all the others in the same instant, as equally as it could.

It is true that this equality could not be totally perfect.  First, because there is no void at all in the new world, it was impossible for all the parts of matter to move in a straight line.  Rather, all of them being just about equal and as easily divertible, they all had to unite in some circular motions.  And yet, because we suppose that God first moved them diversely, we should not imagine that they all came together to turn about a single center, but about many different ones, which we may imagine to be diversely situated with respect to one another....

Thus, in a short time all the parts were arranged in order, so that each was more or less distant from the center about which it had taken its course, according as it was more or less large and agitated in comparison with the others....

Only one must except some which, having been from the beginning much larger than the others, could not be so easily divided or which, having had very irregular and impeding shapes, joined together severally rather than breaking up and rounding off.  Thus, they have retained the form of the third element and have served to compose the planets and the comets, as I shall tell you below....

Imagine, for example, that the points S, E, , and A are the centers of which I speak, that all the matter contained in the space FGGF is a heaven turning about the sun marked S, that all the matter of the space HGGH is another heaven turning about the star marked , and so on for the others.  Thus, there are as many different heavens as there are stars, and, since the number of stars is indefinite, so too is the number of heavens.  Thus also the firmament is nothing other than the breadthless surface separating all the heavens from one another.

Imagine also that the ... speed [of matter in each stellar vortex] decreases little by little from the outside circumference of each heaven to a certain place (such as, for example, to the sphere KK about the sun [S], and to the sphere LL about the star ) and then increases little by little from there to the centers of the heavens because of the agitation of the stars that are found there....

Whence you will be able to understand immediately that the highest planets must move more slowly than the lowest (i.e. those closest to the sun), and that all the planets together move more slowly than the comets, which are nonetheless more distant....

Note finally that, given the manner in which I have said the sun and the other fixed stars were formed, their bodies can be so small with respect to the heavens containing them that even all the circles KK, LL, etc., ... can be considered merely as the points that mark the heavens' center.  In the same way, the new astronomers consider the whole sphere of Saturn as but a point in comparison with the firmament.

On the Origin and the Course of the Planets and Comets in General; and of Comets in Particular

Now, for me to begin to tell you about the planets and comets, consider that, given the diversity of the parts of the matter I have supposed, even though most of them in breaking and dividing by collision with one another have taken the form of the first or second element, there nevertheless does not cease still to be found among them two sorts that had to retain the form of the third element, to wit, those of which the shapes were so extended and so impeding that, when they collided with one another, it was easier for several to join together, and by this means to become larger than to break up and become smaller; and those which, having been from the beginning the largest and most massive of all, could well break and shatter the others in striking them but not in turn be broken or shattered themselves....

[I]f you imagine two rivers that join with one another at some point and then separate again shortly thereafter before their waters (which one must suppose to be very calm and to have a rather equal force, but also to be very rapid) have a chance to mix, then boats or other rather massive and heavy bodies that are borne by the course of the one river will be easily able to pass into the other river, while the lightest bodies will turn away from it and will be thrown back by the force of the water toward the places where it is the least rapid.

For example, if ABF and CDG are two rivers which, coming from two different directions, meet at E and then turn away from there, AB going toward F and CD toward G, it is certain that boat H following the course of river AB must pass through E toward G, and reciprocally boat I toward F, unless both meet at the passage at the same time, in which case the larger and stronger will break the other.  By contrast, scum, leaves of trees, feathers, straw, and other such light bodies that can be floating at A must be pushed by the course of the water containing them, not toward E and toward G, but toward B, where one must imagine that the water is less strong and less rapid than at E, since at B it takes its course along a line that less approaches a straight line.

Moreover, one must consider that not only these light bodies, but also others heavier and more massive can join upon meeting and that, turning then with the water that bears them, several together can compose large balls such as you see at K and L, of which some, such as L go toward E and others, such as K, go toward B, according as each is more or less solid and composed of more or less large and massive parts....

Know also that we should take those that thus tend to range toward the center of any heaven to be the planets, and we should take those that pass across different heavens to be comets.

Now, concerning these comets, one must note first that there must be few of them in this new world in comparison to the number of heavens.  For, even if there were many at the beginning, over the course of time in passing across different heavens almost all of them would have to have collided with one another and broken one another up (just as I have said two boats do when they meet), so that now only the largest could remain.

One must also note that, when they pass thus from one heaven into another, they always push before them some small bit of the matter of the heaven they are leaving and remain enveloped by it for some time until they have entered far enough within the limits of the other heaven.  Once there, they finally loose themselves from it almost all at once and without taking perhaps more time to do so than does the sun in rising at morning on our horizon.  In this way, they move much more slowly when they thus tend to leave some heaven than they do shortly after having entered it.

For example, you see [below] that the comet that takes its course along the line CDQR, having already entered rather far within the limits of the heaven FG, nevertheless when it is at point C still remains enveloped by matter from the heaven FI, from which it comes, and cannot be entirely freed of that matter before it is around point D.  But, as soon as it has arrived there, it begins to follow the course of the heaven FG and thus to move much faster than it did before.  Then, continuing its course from there toward R, its motion must again slow down little by little in proportion as it approaches point Q, both because of the resistance of the heaven FGH, within the limits of which it is beginning to enter, and because, there being less distance between S and D than between S and Q, all the matter of the heaven between S and D (where the distance is smaller) moves faster there, just as we see that rivers always flow more swiftly in the places where their bed is narrower and more confined than in those where it is wider and more extended.

Moreover, one should note that this comet should be visible to those who live at the center of the heaven FG only during the time it takes to pass from D to Q, as you will soon understand more clearly when I have told you what light is.  In the same way, you will see that its motion should appear to viewers to be much faster, its body much greater, and its light much brighter, at the beginning of the time they see it than at the end....

On the Planets in General, and in Particular on the Earth and Moon

Similarly, there are several things to note concerning the planets.  First, even though they all tend toward the center of the heavens containing them, that is not to say thereby that they could ever arrive at those centers.  For, as I have already said above, the sun and the other fixed stars occupy them....

[T]here can be diverse planets, some more and others less distant from the sun, such as here Saturn, Jupiter, Mars, T [Earth], Venus, Mercury.  Of these the lowest and least massive can reach to the sun's surface, but the highest never pass beyond circle K....

[I]t is not simply those that outwardly appear the largest, but those that are the most solid and the most massive in their interior, that should be the most distant....

The ... matter of the heaven must make the planets turn not only about the sun, but also about their own center (except when there is some particular cause that hinders them from doing so), and consequently that the matter must compose around the planets small heavens that move in the same direction as the greater heaven....

For, since the parts of the heaven that are, say, at A move faster than the planet marked T, which they push toward Z, it is evident that they must be diverted by it and constrained to take their course toward B.  I say toward B rather than toward D; for, having inclination to continue their motion in a straight line, they must go toward the outside of the circle ACZN they are describing, rather than toward the center S.

Now, passing thus from A to B, they force the planet T to turn with them about its center.  In turn, this planet in so turning gives them occasion to take their course from B to C, then to D and to A, and thus to form about the planet a particular heaven, with which it must thereafter continue to move from the direction one calls the "occident" [west] toward that which one calls the "orient" [east], not only about the sun but also about its own center.

Moreover, knowing that the planet marked Moon is disposed to take its course along the circle NACZ (just as is the planet marked T) and that it must move faster because it is smaller, it is easy to understand that, wherever it might have been in the heavens at the beginning, it shortly had to tend toward the exterior surface of the small heaven ABCD, and that, once having joined that heaven, it must thereafter always follow its course about T along with the parts of the second element that are at that surface....

I shall not add here how one can find a greater number of planets joined together and taking their course about one another, such as those that the new astronomers have observed about Jupiter and Saturn....

On Weight

Now, however, I would like you to consider what the weight of this earth is; that is to say, what the force is that unites all its parts and that makes them all tend toward its center, each more or less according as it is more or less large and solid.

That force is nothing other than, and consists in nothing other than, the fact that, since the parts of the small heaven surrounding it turn much faster than its parts about its center, they also tend to move away with more force from its center and consequently to push the parts of the earth back toward its center.

Planet T's "small heaven" (circle ABCD) contains earth (circle EFGH) surrounded with layers of water (circle 1234) and air (circle 5678).  The "matter of heaven" fills all the space between the circles 5678 and ABCD.  As this matter circulates, it causes T to turn on its axis, and carries the moon around ABCD.  Inhabitants of T cannot sense that they are spinning in space because they are moving along with everything else in the swirling vortex.

You may find some difficulty in this, in light of my just saying that the most massive and most solid bodies -- such as I have supposed those of the comets to be -- tend to move outward toward the circumferences of the heavens and that only those that are less massive and solid are pushed back toward their centers.  For it should follow therefrom that only the less solid parts of the earth could be pushed back toward its center and that the others should move away from it.  But note that, when I said that the most solid and most massive bodies tended to move away from the center of any heaven, I supposed that they were already previously moving with the same agitation as the matter of that heaven.

For it is certain that, if they have not yet begun to move, or if they are moving less fast than is required to follow the course of this matter, they must at first be pushed by it toward the center about which it is turning.  Indeed, it is certain that, to the extent that they are larger and more solid, they will be pushed with more force and speed....

That the Face of the Heaven of that New World Must Appear to Its Inhabitants Completely Like That of Our World

Having thus explained the nature and the properties of the action I have taken to be light, I must also explain how, by its means, the inhabitants of the planet I have supposed to be the earth can see the face of their heaven as wholly like that of ours.

First, there is no doubt that they must see the body marked S as completely full of light and like our sun, given that that body sends rays from all points of its surface toward their eyes.  And, because it is much closer to them than the stars, it must appear much greater to them....

[Y]ou must ... consider in regard to their arrangement that they can just about never appear in the true place where they are.  For example, that marked appears as if it were in the straight line TB, and the other marked A as if it were in the straight line T4....

And one must suppose those lines TB, T4, and ones like them to be so extremely long in comparison with the diameter of the circle the earth describes about the sun that, wherever the earth is on that circle, the men on it always see the stars as fixed and attached to the same places in the firmament; that is, to use the terms of the astronomers, they cannot observe parallax in the stars.

Regarding the number of those stars, consider also that the same star can often appear in different places because of the different surfaces that divert its rays toward the earth.  Here, for example, that marked A appears in the line T4 by means of the ray A24T and simultaneously in the line Tf by means of the ray A6fT.  In the same way are the objects multiplied that one looks at through glasses or other transparent bodies cut along several faces.

Moreover, regarding their size, consider that they must appear much smaller than they are, because of their extreme distance; for this reason the greater part of them must not appear at all, and others appear only insofar as the rays of several joined together render the parts of the firmament through which they pass a bit whiter and similar to certain stars the astronomers call "nebulous," or to that great belt of our heaven that the poets pretend to be whitened by the milk of Juno....

Moreover, it is very probable that those surfaces, being in a matter that is very fluid and that never ceases to move, should always shake and quiver somewhat, and consequently that the stars one sees through them should appear to scintillate and vibrate, just as ours do, and even, because of their vibration, appear a bit larger.  In this way, the image of the moon appears larger when viewed from the bottom of a lake of which the surface is not very stirred up or agitated, but merely a bit rippled by the breath of some wind.

And, finally, it can happen that, over the course of time, those surfaces change a bit, or indeed even that some of them bend rather noticeably in a short time, even if this is only on the occasion of a comet's approaching them.  By this means, several stars seem after a long time to change a bit in place without changing in size, or to change a bit in size without changing in place.  Indeed, some even begin rather suddenly to appear or to disappear, just as one has seen happen in the real world.

As for the planets and the comets that are in the same heaven as the sun, knowing that the parts of the third element of which they are composed are so large or so joined severally together that they can resist the action of light, it is easy to understand that they must appear by means of the rays that the sun sends toward them and that are reflected from there toward the earth, just as the opaque or obscure objects that are in a room can be seen there by means of the rays that the lamp shining there sends toward them and that return from them toward the eyes of the onlookers....

[T]he motion those planets have about their center is the reason why they twinkle, though much less strongly and in another way than do the fixed stars; because the moon is deprived of that motion, it does not twinkle at all.

As for the comets that are not in the same heaven as the sun, they are far from being able to send out as many rays toward the earth as they could if they were in the same heaven, not even when they are all ready to enter it.  Consequently, they cannot be seen by men, unless perhaps when their size is extraordinary.  The reason for this is that most of the rays that the sun sends out toward them are borne away here and there and effectively dissipated by the refraction they undergo in the part of the firmament through which they pass....

Go to:
  • The Description of a New World, Called The Blazing World (1666), by Margaret Cavendish, Duchess of Newcastle (1623-1673)
  • Conversations on the Plurality of Worlds (1686), by Bernard le Bovier de Fontenelle (1657-1757)
  • Micromegas:  A Tale of Interplanetary Travel (1752), by Voltaire (François Marie Arouet; 1694-1778)
  • An Original Theory or New Hypothesis of the Universe (1750), by Thomas Wright (1711-1786)
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