There is one other point on this subject which I want you to understand. While the atoms are being carried down in a straight line through the void by their own weight, at quite uncertain times and at uncertain intervals they swerve slightly out of their course — just enough for one to be able to say that there has been an alteration in their movement. For if they had not this characteristic of moving out of the direct line, they would all fall downwards like drops of rain through the depths of the void; no collision would take place, no one atom would strike upon another; and so nature would never have produced anything at all.
If by any chance anyone holds the view that heavier bodies, through being carried down more quickly in a straight line through the void, are able to fall from on top on lighter bodies and in this way produce impacts out of which the creative motion could arise, he is quite wrong and has strayed far from the path of true reason. When things fall downwards through water or through thin air, they must indeed accelerate the speed of their fall in proportion to their weights. This is because the corporeal structure of water and the nature of air cannot put a check on each thing equally; the heavier a thing is, the quicker they are mastered and have to give way. The empty void on the other hand cannot possibly offer any resistance to any thing anywhere or at any time: as its whole nature demands, it must always give way. Therefore all bodies, whether their weights are equal or not, must be carried through the peaceful void at an equal speed. And so the heavier bodies will never be able to fall from above on the lighter ones or by themselves produce the blows which give rise to the various motions by which nature carries on her work. Therefore (I must emphasize this point again and again) it is necessary that the atoms swerve slightly; and the swerve must only be the slightest possible; otherwise it will look as though we are assuming oblique motions, a theory which is against the evidence of real facts. For this is something which we see set down plainly before our eyes: that, so far as can be perceived, weights falling straight down from above do not have it in their nature to move obliquely. On the other hand no one has such power of perception as to be able to state that there is absolutely no deviation at all from a perfectly straight course.
Then again, if we assume that all motion always goes on in a continuous chain with new motion always arising out of the old in an absolutely determined order; and if the atoms, by means of this swerve, do not initiate a kind of motion that can break through the decrees of fate so that cause may not ' follow cause to infinity, then how can we explain this free will which we find in living creatures all over the earth? What, I say, is the origin of this faculty of ours which we have wrested from the fates and by which each of us goes where his pleasure leads him, deviating in our motions just as the atoms do at no fixed times or places, but just as our mind takes us? For it is beyond doubt that in these matters it is a man's will that provides the initiative and from it the movements spread through the limbs. You have no doubt observed too that when the barriers are let down at a given moment on a race-course, the strong eager bodies of the horses still cannot burst out into the track as suddenly as their minds in themselves would like to do. This is because the total quantity of matter has to be stirred up together throughout the whole body so that it may then make the collective effort of following the desire of the mind. So you may see that the origin of motion is an act of the intelligence and that this proceeds in the first place from the will of the mind, from it to be passed on further through the whole body and through the limbs. This is not at all the same thing as when we move forward because we are forced to do so by the impulsion of the great strength or great effort of someone else; for in this case it is quite clear that all the matter in the entire body is being pushed forward and hurried along against our will, until the will, operating through the limbs, has regained control. Do you see, then, that, though some external force often drives people on and often compels them to be swept forward headlong against their wills, nevertheless there is something in our breast capable of fighting against this impulse and resisting it? And it is owing to the power of this authority inside ourselves that the whole quantity of matter is sometimes compelled to alter course throughout the body and limbs, and, though pushed forward in one direction, is brought under control and made to settle back again.
You must admit therefore that the same principle holds true of the atoms: that, apart from weight and the blows of one atom on another, there must be another cause for motion, from which comes this power that is born in us, since we see that nothing can be produced out of nothing. It is weight that prevents everything being caused by the blows of one atom on another, as it were by an external force; but it is the minute swerve in the atoms, taking place at no definite time or place, which keeps the mind itself from being governed by an internal necessity in all its actions, and from being as it were subdued by this necessity so as to be merely a passive subject.