This is a hot topic that flares up regularly. If anyone has anything to offer on this to get the ball rolling, please go ahead. Otherwise at some point I'll find a way to phrase the question so we can at least get the ball rolling. The key passages are at the end of the Letter to Menoeceus, one of more Vatican Sayings, and this from Lucretius: (Martin Ferguson Smith, the best and most recent translator, from Lucretius Book 2):
"Moreover, if all movements are invariably interlinked, if new movement arises from the old in unalterable succession, if there is no atomic swerve to initiate movement that can annul the decrees of destiny and prevent the existence of an endless chain of causation, what is the source of this free will possessed by living creatures all over the earth? What, I ask, is the source of this power of will wrested from destiny, which enables each of us to advance where pleasure leads us, and to alter our movements not at a fixed time or place, but at the direction of our own  minds? For undoubtedly in each case it is the individual will that gives the initial impulse to such actions and channels the movements through the limbs.
Have you not observed too that, at the very moment when the starting gates are opened,16 the horses, despite their strength and impatience, cannot burst forward as suddenly as their minds desire? The reason is that the whole mass of matter throughout the whole body must be actuated: only when the whole frame has been actuated can it respond with energy to the eagerness of the mind. So you can see that the initial movement is produced by the mind: it originates from the act of mental  will, and is then diffused through every part of the body."