mon there, that one extended thing cannot penetrate another, rests mainly on prejudice. For whether matter, conceivably and possibly, can enter into matter or not, depends entirely on the sense in which matter is taken. Penetration means the abolition of spatial distinction, and we may hence define matter in such a way that, with loss of spatial distinction, itself would be abolished. If, that is to say, pieces of matter are so one thing with their extensions as, apart from these, to keep no individual difference—then these pieces obviously cannot penetrate; but, otherwise, they may. This seems to me clear, and I will go on to explain it shortly.
It is certain first of all that two parts of one space cannot penetrate each other. For, though these two parts must have some qualities beside their mere extension (Chapter iii.), such bare qualities are not enough. Even if you suppose that a change has forced both sets of qualities to belong to one single extension, you will after all have not got two extended things in one. For you will not have two extended things, since one will have vanished. And, hence, penetration, implying the existence of both, has become a word without meaning. But the case is altered, if we consider two pieces of some element more concrete than space. Let us assume with these, first, that their other qualities, which serve to divide and distinguish them, still depend on extension—then, so far, these things still cannot penetrate each other. For, as before, in the one space you would not have two things, since (by the assumption) one thing has lost separate existence. But now the whole question is whether with matter this assumption is true, whether in Nature, that is, qualities are actually so to be identified with extension. And, for myself, I find no reason to think that this is so. If in two parts of one extended there are distinctions sufficient to individualize, and
