On the Nature of Things (De Rerum Natura)

While, on the contrary, the nature of void is ever without weight. Therefore, when a thing is just as large, yet is found to be lighter, it proves sure enough that it has more of void in it; while on the other hand that which is heavier shews that there is in it more of body and that it contains within it much less of void. Therefore, that which we are seeking with keen-sighted reason exists sure enough, mixed up in things; and we call it void.
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And herein I am obliged to forestall this point which some raise, lest it draw you away from the truth. The waters, they say, make way for the scaly creatures fish as they press on, and open liquid paths, because the fish leave room behind them, into which the yielding waters may stream; thus other things too may move and change place among themselves, although the whole sum be full. This, you are to know, has been taken up wholly on false grounds. For on what side, I ask, can the scaly creatures move forwards, unless the waters have first made room? Again, on what side can the waters give place, so long as the fish are unable to go on? Therefore, you must either strip all bodies of motion or admit that in things void is mixed up from which every thing gets its first start in moving.