Lucretius, answered from the texts and cited to the page.
It is both, and I would not have it otherwise. The swerve is a thing the physics requires before it is a thing the mind requires. Take the physics first. First-bodies fall through the void by their own weight, and the void gives place to all alike, so that heavy and light move at equal rate; no heavier body can ever fall upon a lighter from above and strike out the blows that make the world.1
If they went only straight down they would drop like rain, no collision would come to be, and nature would never have brought anything to birth.2 So at times quite undetermined and at spots undetermined they push a little from their path, just so much as you might call a change of motion.2 I hold it to the very least, lest we seem to imagine a sideways movement and the plain truth refute it; for we see that bodies, as far as in them lies, fall headlong from above.3
But that nothing at all swerves from the straight, what sense can descry that? The swerve is too small for the eye, and so it must be argued, as I argue the atoms themselves, from what we do see to what we cannot.3 Now the second work. If every motion is linked to an old one in order determined, and the first-beginnings never by swerving make a start of movement to break the decrees of fate, so that cause follows cause from infinite time, then whence comes this free will for living things over all the earth?
Whence is it wrested from fate, this will by which we go where pleasure leads each one, and turn in our motions at no fixed time and in no fixed place, but where the mind has carried us?3 The beginning is in the will, and from the will the motion floods through the limbs; you see it when the barriers fly open and the horses' eagerness cannot burst out at once as their mind desires, for all the matter of the body must be roused through every limb to follow the longing of the heart.3
And it is not as when another's blow drives us on against our will. Many a man is hurried headlong by a force outside, and yet there is something in our breast that can fight against it and withstand it.4 Therefore in the seeds themselves you must allow another cause of motion besides blows and weights, whence this power is born in us, since nothing comes from nothing.
Weight forbids everything to come about except by blows, as by a force from without. That the mind itself feels no necessity within, and is not forced like a conquered thing to bear and suffer, is brought about by the tiny swerve of the first-beginnings, in no determined place and at no determined time.4 As to the ancients seeing a crack: the Greeks disputed it, and some scoffed.
But a cause too small to be seen is still a cause. A man who calls the swerve a flaw must say what, if not this, ever began a collision, or ever began a will. The alternative is that nothing began at all.
The first-beginnings fall through the void by their weight at equal speed, and if they only fell they would never meet; nature would have made nothing. So at times quite undetermined, and at places undetermined, they push a little from their path, only so much as can be called a change of course. That little is what frees the mind from the chain of cause after cause: the will that sets the limbs moving is not pushed from outside, it begins in the breast, and the swerve is where that beginning lives in the atoms.5
Without it there is no collision and no freedom; with it, nothing in the physics is broken, because nothing in the physics ever said the atoms fall in straight lines forever.6789
the heavier will not then ever be able to fall on the lighter from above, nor of themselves bring about the blows, which make diverse the movements, by which nature carries things on.On the Nature of Things, pp. 72–73
all things would fall through the deep void like drops of rain, nor could collision come to be, nor a blow brought to pass for the first-beginnings: so nature would never have brought aught to being.On the Nature of Things, pp. 72–73
whence comes this free will for living things all over the earth, whence, I ask, is it wrested from fate, this will whereby we move forward, where pleasure leads each one of us, and swerve likewise in our motions neither at determined times nor in a determined direction of place, but just where our mind has carried us?On the Nature of Things, p. 74
that the very mind feels not some necessity within in doing all things, and is not constrained like a conquered thing to bear and suffer, this is brought about by the tiny swerve of the first-beginnings in no determined direction of place and at no determined time.On the Nature of Things, pp. 75–76
Do you not then now see that, albeit a force outside pushes many men and constrains them often to go forward against their will and to be hurried away headlong, yet there is something in our breast, which can fight against it and withstand it? And at its bidding too the store of matter is constrained now and then to turn throughout the limbs and members, and, when pushed forward, is reined Book II, Ul.On the Nature of Things, pp. 75–76
And you descry, too, thunderbolts flying crosswise through the rain; now from this side, now from that the fires burst from the clouds and rush together; the force of flame everywhere falls towards the earth.On the Nature of Things, pp. 72–73
But when they suppose that the gods have appointed all things for the sake of men, they are seen in all things to fall exceeding far away from true reason.On the Nature of Things, pp. 71–72
For this we see plain and evident, that bodies, as far as in them lies, cannot travel sideways, since they fall headlong from above, as far as you can descry.On the Nature of Things, p. 74
Next, what speed of movement is given to the firstbodies of matter, you may learn, Memmius, in a few words from this. First, when dawn strews the land with new light, and the diverse birds flitting through the distant woods across the soft air fill the place with their clear cries, we see that it is plain and evident for all to behold 70 Book II, ll.On the Nature of Things, pp. 70–71
Mocked by Cicero, defended by the school, argued over by the nineteenth-century scholars Masson reports; the one place where the poem's own physics is turned against it.