Lucretius, De Rerum Natura
William Ellery Leonard, Ed.

     Now, how it comes that we,
Whene'er we wish, can step with strides ahead,
And how 'tis given to move our limbs about,
And what device is wont to push ahead
This the big load of our corporeal frame,
I'll say to thee- do thou attend what's said.
I say that first some idol-films of walking
Into our mind do fall and smite the mind,
As said before. Thereafter will arises;
For no one starts to do a thing, before
The intellect previsions what it wills;
And what it there pre-visioneth depends
On what that image is. When, therefore, mind
Doth so bestir itself that it doth will
To go and step along, it strikes at once
That energy of soul that's sown about
In all the body through the limbs and frame-
And this is easy of performance, since
The soul is close conjoined with the mind.
Next, soul in turn strikes body, and by degrees
Thus the whole mass is pushed along and moved.
Then too the body rarefies, and air,
Forsooth as ever of such nimbleness,
Comes on and penetrates aboundingly
Through opened pores, and thus is sprinkled round
Unto all smallest places in our frame.
Thus then by these twain factors, severally,
Body is borne like ship with oars and wind.
Nor yet in these affairs is aught for wonder
That particles so fine can whirl around
So great a body and turn this weight of ours;
For wind, so tenuous with its subtle body,
Yet pushes, driving on the mighty ship
Of mighty bulk; one hand directs the same,
Whatever its momentum, and one helm
Whirls it around, whither ye please; and loads,
Many and huge, are moved and hoisted high
By enginery of pulley-blocks and wheels,
With but light strain.