1. By virtue of their inherent weight (pondus), (a) all bodies have a natural downward motion, which (b) in a void is equally fast in all, regardless of their weights (an anticipation of Galileo?). Given this, if weight were the only factor to consider, there would never be any collisions among bodies. Consequently, it is necessary to posit that bodies “incline a little,” though “not more than the least possible” (245). On the face of it, this strikes me as a valid scientific argument. We have reason to ascribe one kind of natural motion to bodies. But this motion appears to be inadequate to account for the phenomena, and so an additional factor is posited. So far as I can see, Lucretius makes no claim about the modality/causality of these small “oblique motions.” He does ascribe a priority and naturalness to the downward motion of bodies: “heavy things, as far as it lies in them, [quantum in se est] cannot move obliquely” (247). And he stresses the lack of predictability of oblique motions (they occur “at times quite uncertain and uncertain places”: 218-9). Yet this is consistent with these motions being truly chance events, uncaused by anything at all, or with them being ascribable to some unknown property of atoms.

2. Although it appears to be a continuation of the same line of argument, the caveat regarding free or voluntary motion at 251ff makes a very different claim. Here the problem is not the supposition that all natural motion is downward motion, but that all motion is determined by prior causes, according to a “definite order,” or the “decrees of fate” (fati foedera). If this were the case, Lucretius argues, there would be no room for voluntary agency. And so, we must have the ability to incline our motion wherever the mind leads (declinamus item motus… ubi ipsa tulit mens).

In this case, Lucretius is not gesturing at a uncaused, or chance, event, but one that is initiated by the agent (“it is his own will in each that begins these things, and from the will movements go rippling through the limbs”). Furthermore, he explicitly cites the motive that is relevant in accounting for such action: “whence I say is this will [voluntas] wrested from the fates by which we proceed whither pleasure [voluptas] leads each, inclining also our motion not at fixed times and fixed places, but just where our mind leads us?”

The most straightforward reading of this passage would be to see it as positing some kind of sui generis agent causation, but I’m not convinced that this is the best reading.