Aristotle's theory of place and his concept of the void are aspects of the same problem, namely, the spatial and physical conditions of motion; they should be studied together. Modern scientists, for their part, have mostly abandoned the Aristotelian theories in hand, preferring, because of their mathematical leaning, to consider motion under the conditions of space rather than place. Nevertheless, the basic ideas and problems are much the same either way, and we shall therefore not only present Aristotle's notion of place and the void but also take a brief comparative look at the modern notion of space.
1.The Problem of Place8
Everybody has some idea of "place" or what it means "to be in a place." Everything all around is localized, is "somewhere." The fact is particularly noticed when we see the displacement of something. Where there was water in a pitcher, there is now milk or wine or just plain air. The contents changed, the place remained. The existence of place is further underscored by local motion, which, both by definition and experience, is precisely a going from one place to another. And if we advert to the behavior of the elements, water, air, etc., we discover something else about place. These elements clearly display a natural movement in a certain direction, up or down or between. Place, every place, has therefore a power of attraction that is proper to it, and not all places wield the same specific attraction. Such, in the main, are the observations by which Aristotle introduces his discussion on place, and having made them, he plunges at once into the not inconsiderable difficulties as to its nature. What, he asks, is place? It cannot be a body, for then there should be two bodies in the same place. Nor can it be part of the body it contains, since this body may be removed and the place left behind. And what happens when a body increases? or grows? Does its place grow, too? This, as we shall see, seems inadmissible, at least in Aristotle's view of the matter. In short, place presents a number of difficult questions. A most familiar thing, it has much to baffle search and scrutiny. Aristotle discusses these difficulties along with related questions in the first three chapters of Book IV. Finally, in the beginning of chapter 4, he brings his preliminary inquiry to a close with an enumeration of the things that appear to be essential characteristics of place. These, in outline, are three. 1) Place is the first limit or surrounding of the body it localizes; this is a fact of ordinary experience. 2) Place is independent of the thing it contains, hence separable from the thing. 3) Place is physically determined; there is an "up" and a "down" to place, and these positions have their distinctive power of attraction. These are the assumptions from which Aristotle proceeds to evolve the definition of place.
2. Definition of Place9
In his effort to settle on a definition Aristotle considers four possibilities, of which the first three are found in error and so discarded.
Place, according to one suggestion, would be the form, not the substantial form but the exterior configuration of a body, its shape or "figure" (fourth species of quality). But this is impossible because a body and its shape are one piece and the shape disappears with the body. Secondly, place might seem to be the matter of the contained body. The same reason as before militates against this because matter is neither separable from a body nor contains it; place, however, is separable from a body and contains it. Note again, however, that Aristotle is not speaking of prime matter, but of matter in the Platonic sense, that is, space considered as an undefined reality, a kind of receptacle for bodies which successively occupy it. Thirdly, place is thought to be the interval between the extremities of container and contained, because the contained may change while the container remains the same. Thus place would be an entity independent of bodies, an empty space. Aristotle rejects this view because the interval does not exist by itself but as an accident of bodies which successively fill the container.
Rejecting, as we have said, all three of the aforesaid opinions, Aristotle concludes that place is the limit (boundary) of the containing body, or in the Latin idiom, the "terminus corporis continentis." This boundary does indeed give the appearance of being a surrounding surface existing independently of a body. And well it might, for it is not a mere abstraction; it is something real, with properties that are real.
Place is immovable. - One important doubt concerning place remains to be satisfied. If place is the enveloping container of a body, does place change abode with a body, as when a vessel is carried away with its contents? Or, - and this comes to the same thing - suppose that the contents remain unmoved but the surrounding bodies are displaced. A notable instance of the latter is a stationary body surrounded by flowing water, say a boat moored in a river. The river flows ceaselessly by. Shall we say that the place of the boat is constantly changing?
Aristotle will not have it so. Place, he insists, is immovable, as experience itself seems to indicate. As for the boat anchored in the river, its true place is the whole river, since as a whole the river is immovable or fixed. This is a significant assertion. Place, it turns out, is determined not so much by the immediate surrounding of a body as by its ultimate environment. Taking this position, Aristotle is undoubtedly tempering his previous remarks on the meaning of place. The immediate layer or container, it now becomes clear, is only a derivative or relative principle of localization. The radical principle of place, the one that positions and immobilizes the limits of the immediate container, this is the outermost layer or shell of the universe, which was thought to be immovable. And this outermost, motionless layer of the universe is the indispensable key to the traditional definition, namely, that place is "the immovable limit of the immediate container," or as the Latin has it,
terminus immobilis continentis primum.10
Obviously, the ultimate, motionless layer of the heavens is not the immediate container of all bodies, but in the view of the ancients it is, to repeat, the layer or shell in relation to which the limits of the immediate container are fixed and immovable, hence always the same.11
3. Function of Place in Aristotelian Cosmology
What, more precisely, is this ultimate layer or primary container? The answer is provided in the cosmology, or rather the astronomy of the ancients, since this is clearly the background of Aristotle's theory of place. In the ancient theory the ultimate or, from another point of view, the first container is the sphere of the fixed stars, the last of the celestial spheres. From this the extreme positions of place, that is, up and down, were determined. Whatever was toward the circumference of this sphere was "up," and what was toward the center (the earth) was "down," while the intermediate places were reckoned from these extremes. Thus, each thing's natural place was defined in relation to the outermost sphere, and changes in things were explained, in part at least, as a seeking or finding of their natural place.
It should be apparent, therefore, that place has a central role in Aristotle's theory of the physical universe. The basic movement of the four elements is, as just indicated, a response to the attraction of their natural place. What is light tends naturally toward the higher places; what is heavy, toward the lower. Hence the importance of the theory of place. Remember that local motion is the cardinal motion, the one that commands all other changes and transformations in the sublunary world. But local motion itself is controlled by place, or the attraction of place.
Consequently, place as envisaged by Aristotle is nothing less than the foundation of all cosmic movement, the masterwheel of celestial as well as terrestrial mechanics. Such is the role, the pivotal role, of place in Aristotle's scheme of the material universe.
This theory of place, however, still leaves Aristotle with a twofold problem. What of the last (or first) sphere? Is it also localized? And if not, how understand the motion of a body that is not in a place?
a) According to the ancients, with whom Aristotle is here in accord, the first heaven or sphere is not in any place, since there is nothing further to limit or contain it.
b) But then, what explanation is there for the uniform motion, or so it seems, of the heavens? This point caused Aristotle's commentators no end of perplexity. Averroes thought that the fixed position of the center, the earth, served somehow to place the celestial spheres. St. Thomas, preferring Themistius' solution, holds for localization of the parts in relation to one another. Thus, there can be motion in each of the parts, but not in the sphere taken as a whole, since this, by the ancients, is strictly speaking not in a place.