On the nature of things (books 4 - 6)   

The smelling object is. For, dallying on
Along the winds, the particles cool off,
And then the scurrying messengers of things
Arrive our senses, when no longer hot.
So dogs oft wander astray, and hunt the scent.

Now mark, and hear what objects move the mind,
And learn, in few, whence unto intellect
Do come what come. And first I tell thee this:
That many images of objects rove
In many modes to every region round-
So thin that easily the one with other,
When once they meet, uniteth in mid-air,
Like gossamer or gold-leaf. For, indeed,
Far thinner are they in their fabric than
Those images which take a hold on eyes
And smite the vision, since through body's pores
They penetrate, and inwardly stir up
The subtle nature of mind and smite the sense.
Thus, Centaurs and the limbs of Scyllas, thus
The Cerberus-visages of dogs we see,
And images of people gone before-
Dead men whose bones earth bosomed long ago;
Because the images of every kind
Are everywhere about us borne- in part
Those which are gendered in the very air
Of own accord, in part those others which
From divers things do part away, and those
Which are compounded, made from out their shapes.
For soothly from no living Centaur is
That phantom gendered, since no breed of beast
Like him was ever; but, when images
Of horse and man by chance have come together,
They easily cohere, as aforesaid,
At once, through subtle nature and fabric thin.
In the same fashion others of this ilk
Created are. And when they're quickly borne
In their exceeding lightness, easily
(As earlier I showed) one subtle image,
Compounded, moves by its one blow the mind,
Itself so subtle and so strangely quick.

That these things come to pass as I record,
From this thou easily canst understand:
So far as one is unto other like,
Seeing with mind as well as with the eyes
Must come to pass in fashion not unlike.
Well, now, since I have shown that I perceive
Haply a lion through those idol-films
Such as assail my eyes, 'tis thine to know
Also the mind is in like manner moved,
And sees, nor more nor less than eyes do see
(Except that it perceives more subtle films)
The lion and aught else through idol-films.
And when the sleep has overset our frame,
The mind's intelligence is now awake,
Still for no other reason, save that these-
The self-same films as when we are awake-
Assail our minds, to such degree indeed
That we do seem to see for sure the man
Whom, void of life, now death and earth have gained
Dominion over. And Nature forces this
To come to pass because the body's senses
Are resting, thwarted through the members all,