ON OCCUPYING SPACE
H. W. B. Joseph · Mind · 1919
THE object of this paper is chiefly critical.I wish to explain certain difficulties whioh I seem to find in the relation of bodies to space.But at the end I shall suggest that a sense of some suoh difficulties may underlie language used by Plato in a well-known passage of the Timaeus, 50-52.How far my difficulties have been already expressed by others, I do not know; and should be grateful to any reader who would point out to me an exposition of them.Fundamentally, the difficulty may be put this way: What is meant by saying that a body occupies space ?Connected with it is the question, what distinguishes a body from a geometrical solid of the same outline, or, What is solidity ?But I will begin by asking a question slightly different, in which I find the problem more easy to indicate: What happens when a body moves ?When a body moves, it comes to be in a new place.Now I think we commonly imagine that to put a body in a place is like putting it in a box, and that there is no more difficulty about the one than the other.This is not so.To put a ball in a box is to bring it into new space-relations to other bodies; in particular, to the box.I am not concerned with the space-relations of bodies to one another, but of a body to the space whioh it occupies.Now if anything is unextended, I cannot occupy a place with it; I cannot put a sound or a fear in a new, or any, place.The moving thing is already an extended thing, occupying a place, i.e., a certain portion of space.When it moves, that portion of space does not move.Does the body then, if I may so express myself, carry its extension with it, or not ?If not, it would appear that in the act of motion it ceases to be extended; if yes, that one extension is in another.I am aware that some will denounce this language, and say that I ought not to speak of an extension, but only of an extended thing; and that by so putting it, the difficulty disappears.I hardly think so, and I am content to use the phrase, if it will create a sense of the difficulty.Let me put it in another way.Imagine a geometrical solid, discriminated by the colour of its surface.A coloured surface has no thickness, though the body whose surface is coloured may have.Now if the position of this coloured surface shifted, the geometrical solid would appear to move.Apart from problems about continuity (with which I am not concerned), I find no difficulty here, for there is no space-filling body; what shifts its place is a mere outline, which carries, as it were, no extension with it.Doubtless there are physical objections to the notion of a coloured