On the nature of things (books 1 - 3)   

On any side, at any time, to aught
Oppose resistance, but will ever yield,
True to its bent of nature. Wherefore all,
With equal speed, though equal not in weight,
Must rush, borne downward through the still inane.
Thus ne'er at all have heavier from above
Been swift to strike the lighter, gendering strokes
Which cause those divers motions, by whose means
Nature transacts her work. And so I say,
The atoms must a little swerve at times-
But only the least, lest we should seem to feign
Motions oblique, and fact refute us there.
For this we see forthwith is manifest:
Whatever the weight, it can't obliquely go,
Down on its headlong journey from above,
At least so far as thou canst mark; but who
Is there can mark by sense that naught can swerve
At all aside from off its road's straight line?

Again, if ev'r all motions are co-linked,
And from the old ever arise the new
In fixed order, and primordial seeds
Produce not by their swerving some new start
Of motion to sunder the covenants of fate,
That cause succeed not cause from everlasting,
Whence this free will for creatures o'er the lands,
Whence is it wrested from the fates,- this will
Whereby we step right forward where desire
Leads each man on, whereby the same we swerve
In motions, not as at some fixed time,
Nor at some fixed line of space, but where
The mind itself has urged? For out of doubt
In these affairs 'tis each man's will itself
That gives the start, and hence throughout our limbs
Incipient motions are diffused. Again,
Dost thou not see, when, at a point of time,
The bars are opened, how the eager strength
Of horses cannot forward break as soon
As pants their mind to do? For it behooves
That all the stock of matter, through the frame,
Be roused, in order that, through every joint,
Aroused, it press and follow mind's desire;
So thus thou seest initial motion's gendered
From out the heart, aye, verily, proceeds
First from the spirit's will, whence at the last
'Tis given forth through joints and body entire.
Quite otherwise it is, when forth we move,
Impelled by a blow of another's mighty powers
And mighty urge; for then 'tis clear enough
All matter of our total body goes,
Hurried along, against our own desire-
Until the will has pulled upon the reins
And checked it back, throughout our members all;
At whose arbitrament indeed sometimes
The stock of matter's forced to change its path,
Throughout our members and throughout our joints,
And, after being forward cast, to be
Reined up, whereat it settles back again.
So seest thou not, how, though external force
Drive men before, and often make them move,
Onward against desire, and headlong snatched,
Yet is there something in these breasts of ours
Strong to combat, strong to withstand the same?-
Wherefore no less within the primal seeds