Ad

Ethics  (1677)

by  Baruch Spinoza

Rarefaction and condensation can be clearly and distinctly conceived from the mere fact that parts of a body may move away from, or toward, one another. Therefore (Ax. 4) they will not occupy either more or less space. For if the parts of a body -- say, a sponge -- by moving toward one another expel the bodies with which its interstices are filled, this in itself will make that body more dense, and its parts will not thereby occupy less space than before (Ax. 4). And if again the parts move away from one another and the gaps are filled by other bodies, there will be rarefaction, but the parts will not occupy more space. And this, which we clearly perceive with the aid of our senses in the case of a sponge, we can conceive with the unaided intellect in the case of all bodies, although their interstices completely escape human sense-perception. Therefore rarefaction and condensation are clearly and distinctly conceived by us, etc. Q.E.D.