The Existence of Other Worlds is Possible
Now we have finished the chapters in which Aristotle undertook to prove that a plurality of worlds is impossible, and it is good to consider the truth of this matter without considering the authority of any human but only that of pure reason. I say that, for the present, it seems to me that one can imagine the existence of several worlds in three ways. One way is that one world would follow another in succession of time, as certain ancient thinkers held that this world had a beginning because previous to this all was a confused mass without order, form, or shape. Thereafter, by love or concord, this mass was disentangled, formed, and ordered, and thus was the world created. And finally after a long time this world will be destroyed by discord and will return to the same confused mass, and again, through concord another world will then be made. Such a process will take place in the future an infinite number of times, and it has been thus in the past. But this opinion is not touched upon here and was reproved by Aristotle in several places in his philosophical works. It cannot happen in this way naturally, although God could do it and could have done it in the past by His own omnipotence, or He could annihilate this world and create another thereafter. And, according to St. Jerome, Origen used to say that God will do this innumerable times.
Another speculation can be offered which I should like to toy with as a mental exercise. This is the assumption that at one and the same time one world is inside another so that inside and beneath the circumference of this world there was another world similar but smaller. Although this is not in fact the case, nor is it at all likely, nevertheless, it seems to me that it would not be possible to establish the contrary by logical argument; for the strongest arguments against it would, it seems to me, be the following or similar ones. First, if there were another world inside our world, it would follow that our earth is where it is by constraint, because inside this earth and beneath its circumference toward its center would be another heaven and other elements, etc. Also, the earth of the second world would be absolutely massive and at the center of both worlds; and the earth of our world would be empty and concave and neither the whole earth nor any part of it would be at the center. Thus, since their natural places are different, it follows from what is said in Chapter Seventeen that these two worlds are of different form so that the world beneath us and this our world would be dissimilar, etc. Also, all natural bodies are limited in bigness and smallness, for the size of a man could diminish or grow so much that he would no longer be a man, and the same with all bodies. So, the world we have imagined inside our own world and beneath its circumference would be so small that it would not be a world at all, for our sun would be more than 2,000 times the size of the other and each of our stars would be larger than this imaginary world. To pursue our thought, one could dig in the ground deep enough to reach the earth of the other world beneath ours. This is an untenable absurdity. Also, we should have to posit two Gods, one for each world, etc. Likewise, we might assume another world like our own to exist in the moon or some other star, etc. Or we could imagine another world above and another beneath the one which is under our world, etc.
To show that these and similar speculations do not preclude the possibility of such a thing, I will posit, first of all, that every body is divisible into parts themselves endlessly divisible, as appears in Chapter One; and I point out that large and small are relative, and not absolute, terms used in comparisons. For each body, however small, is large with respect to the thousandth part of itself, and any body whatsoever, however large, would be small with respect to a larger body. Nor does the larger body have more parts than the smaller, for the parts of each are infinite in number. Also from this it follows that, were the world to be made between now and tomorrow 100 or 1,000 times larger or smaller than it is at present, all its parts being enlarged or diminished proportionally, everything would appear tomorrow exactly as now, just as though nothing had been changed. And, if a stone in a quarry had a small opening in it or a concavity full of air, it is not necessary to say that this stone is outside its natural place. Likewise, if there were a concavity the size of an apple full of air at the earth's center, it would not follow that the earth was out of its natural place nor that it was there by violence. Also, if such concavity were to become a bit larger and then still larger until it became very large, we could not place a limit upon this growth at which point one could say the earth would be out of its natural place, precisely because large and small are relative terms, as we have already said. Therefore, for the earth to be in its natural place, it is enough that the center of its weight should be the center of the world, regardless of the concavity inside the earth, provided that it be held firmly together. And this is the answer to the first argument; for, if a world were enclosed within a concavity inside our earth, nevertheless our earth would be in its natural place since the center of the world would be the middle or center of its weight. A propos, I say further that, according to Scripture, water is above the heavens or the firmament; whence the psalm says [Ps. 103:2-3]: Who stretchest out the heavens, etc., Who coverest these heavens with water. And, elsewhere: Bless the Lord, ye waters that are above the heavens [see Dan. 3:60]. And if this water were not heavy in substance if not in fact, then it would not be water. For this reason it is said to be solid and as though frozen or solidified and is called the glacial or the crystalline heavens. Accordingly, this heaven or this water is in its natural place, in spite of the fact that all the other heavenly spheres and elements are enclosed within the concavity of this sphere, for it is solid and the center of its weight is the center of the world. To the second argument I reply that, even if this earth were hollow and concave, nevertheless it would be in the center of the world or worlds, just as though this were its proper place, taking place in the sense of the second member of the distinction made in reply to an argument in Chapter Seventeen. From this it appears that our earth and the earth of the other world within it would be in the same place. To the third argument, which stated that all natural bodies are limited in quantity, I say that in this world they are limited to one quantity or size and that in another world they would be fixed at other limits, for large and small, as we have said, are relative terms which do not mean variation or difference in form. Accordingly, we see men–all of the same form–larger in one region and smaller in another. To the fourth argument, where it was stated that one could dig deep enough into the earth, etc., I answer that nature would not permit this, any more than one could naturally approach the sky close enough to touch it. To the fifth argument, regarding the possibility of two Gods, it does not follow; for one sovereign God would govern all such worlds, but it is possible that additional intelligences would move the heavenly bodies of one world and other intelligences the heavens of the other world. To the sixth argument, where it was said that by analogy one could say there is another world inside the moon, and to the seventh, where it was posited that there are several worlds within our own and several outside or beyond which contain it, etc., I say that the contrary cannot be proved by reason nor by evidence from experience, but also I submit that there is no proof from reason or experience or otherwise that such worlds do exist. Therefore, we should not guess nor make a statement that something is thus and so for no reason or cause whatsoever against all appearances; nor should we support an opinion whose contrary is probable; however, it is good to have considered whether such opinion is impossible.
The third manner of speculating about the possibility of several worlds is that one world could be entirely outside the other in an imagined space, as Anaxagoras held. This solitary type of other world is refuted here by Aristotle as impossible. But it seems to me that his arguments are not clearly conclusive, for his first and principal argument states that, if several worlds existed, it would follow that the earth in the other world would tend to be moved to the center of our world and conversely, etc., as he has loosely explained in Chapters Sixteen and Seventeen. To show that this consequence is not necessary, I say in the first place that, although up and down are said with several meanings, as will be stated in Book II, with respect to the present subject, however, they are used with regard to us, as when we say that one-half or part of the heavens is up above us and the other half is down beneath us. But up and down are used otherwise with respect to heavy and light objects, as when we say the heavy bodies tend downward and the light tend upward. Therefore, I say that up and down in this second usage indicate nothing more than the natural law concerning heavy and light bodies, which is that all the heavy bodies so far as possible are located in the middle of the light bodies without setting up for them any other motionless or natural place. This can be understood from a later statement and from an explanation in the fourth chapter, where it was shown how a portion of air could rise up naturally from the center of the earth to the heavens and could descend naturally from the heavens to the center of the earth. Therefore, I say that a heavy body to which no light body is attached would not move of itself; for in such a place as that in which this heavy body is resting, there would be neither up nor down because, in this case, the natural law stated above would not operate and, consequently, there would not be any up or down in that place. This can be clarified by what Aristotle says in Book Four of the Physics, namely, that in a void there is no difference of place with respect to up or down. Therefore, Aristotle says that a body in a vacuum would not move of itself. In the eleventh chapter of this first book it appears, according to Aristotle, that, since nothing is lower than the center of the earth, nothing is or can be higher than the circumference or the concavity of the lunar sphere, the place proper to fire, as we have often said. Thus, taking up in the second sense above, beyond or outside of this circumference or heaven there is no up nor down. From this it follows clearly that, if God in His infinite power created a portion of earth and set it in the heavens where the stars are or beyond the heavens, this earth would have no tendency whatsoever to be moved toward the center of our world. So it appears that the consequence stated above by Aristotle is not necessary. I say, rather, that, if God created another world like our own, the earth and the other elements of this other world would be present there just as they are in our own world. But Aristotle confirms his conclusion by another argument in Chapter Seventeen and it is briefly this: all parts of the earth tend toward a single natural place, one in number; therefore, the earth of the other world would tend toward the center of this world. I answer that this argument has little appearance of truth, considering what is now said and what was said in Chapter Seventeen. For the truth is that in this world a part of the earth does not tend toward one center and another part toward another center, but all heavy bodies in this world tend to be united in one mass such that the center of the weight of this mass is at the center of this world, and all the parts constitute one body, numerically speaking. Therefore, they have one single place. And if some part of the earth in the other world were in this world, it would tend toward the center of this world and become united with the mass, and conversely. But it does not have to follow that the portions of earth or of the heavy bodies of the other world, if it existed, would tend to the center of this world because in their world they would form a single mass possessed of a single place and would be arranged in up and down order, as we have indicated, just like the mass of heavy bodies in this world. And these two bodies or masses would be of one kind, their natural places would be formally identical, and likewise the two worlds.
In Chapter Twenty Aristotle mentions another argument from what was said in the Metaphysics-namely, that there cannot be more than one God and, therefore, it seems there can be only one world. I reply that if God is infinite in his immensity, and, if several worlds existed, no one of them would be outside Him nor outside His power; but surely other intelligences would exist in one world and others in the other world, as already stated. And my reply to this argument is given more fully in Chapter Twenty. He argues again in Chapters Twenty-two and Twenty-three of which the purport is briefly this: this world is composed of all the matter available for the constitution of a world, and outside this world there can be no body or matter whatsoever. So it is impossible that another world exists. In reply, I say in the first place, that, assuming that all the matter now existing or that has ever existed is comprised in our world, nevertheless, in truth, God could create ex nihilo new matter and make another world. But Aristotle would not admit this. Thus, I say, secondly, that, assuming that nothing could be made save from matter already existing and considering the replies we have given to Aristotle's first arguments regarding this problem-arguments whose substance he repeats and employs here in the present case-nonetheless he does not prove that another or more than one world besides our own could not now exist or may not always have existed, just as he states this world of ours to exist without beginning or end. He argues again in Chapter Twentyfour that outside this world there is no place or plenum, no void, and no time; but he proves this statement by saying that outside this world there can be no body, as he has shown by the reasoning above to which my position could be strengthened or restated otherwise; for, if two worlds existed, one outside the other, there would have to be a vacuum between them for they would be spherical in shape; and it is impossible that anything be void, as Aristotle proves in the fourth book of the Physics. It seems to me and I reply that, in the first place, the human mind consents naturally, as it were, to the idea that beyond the heavens and outside the world, which is not infinite, there exists some space whatever it may be, and we cannot easily conceive the contrary. It seems that this is a reasonable opinion, first of all, because, if the farthest heaven on the outer limits of our world were other than spherical in shape and possessed some high elevation on its outer surface similar to an angle or a hump and if it were moved circularly, as it is, this hump would have to pass through space which would be empty-avoid-when the hump moved out of it. Now, if we assumed that the outermost heaven was not thus shaped or that nature could not make it thus, nevertheless, it is certainly possible to imagine this and certain that God could bring it about. From the assumption that the sphere of the elements or of all bodies subject to change contained within the arch of the heavens or within the sphere of the moon were destroyed while the heavens remained as they are, it would necessarily follow that in this concavity there would be a great expanse and empty space. Such a situation can surely be imagined and is definitely possible although it could not arise from purely natural causes, as Aristotle shows in his arguments in the fourth book of the Physics, which do not settle the matter conclusively, as we can easily see by what is said here. Thus, outside the heavens, then, is an empty incorporeal space quite different from any other plenum or corporeal space, just as the extent of this time called eternity is of a different sort than temporal duration, even if the latter were perpetual, as has been stated earlier in this chapter. Now this space of which we are talking is infinite and indivisible, and is the immensity of God and God Himself, just as the duration of God called eternity is infinite, indivisible, and God Himself, as already stated above. Also, we have already declared in this chapter that, since our thinking cannot exist without the concept of transmutation, we cannot properly comprehend what eternity implies; but, nevertheless, natural reason teaches us that it does exist. In this way the Scriptural passage, Job 26:, which speaks about God can be understood: Who stretchest out the north over the empty place. Likewise, since apperception of our understanding depends upon our corporeal senses, we cannot comprehend nor conceive this incorporeal space which exists beyond the heavens. Reason and truth, how ever, inform us that it exists. Therefore, I conclude that God can and could in His omnipotence make another world besides this one or several like or unlike it. Nor will Aristotle or anyone else be able to prove completely the contrary. But, of course, there has never been nor will there be more than one corporeal world, as was stated above.
Le Livre du ciel et du monde, ed. by Albert D. Menut and Alexander J. Denomy. Translated with an introduction by Albert D. Menut (Madison: University of Wisconsin Press, 1968), pp. 167-179. Reprinted by permission of The University of Wisconsin Press in E. Grant, Science & Religion. From Aristotle to Copernicus (Baltimore: The John Hopkins University Press, 2004), pp. 271-278.