On the nature of things (books 4 - 6)   

And rising out, along their bending path
They're torn asunder, nor have gateways straight
Wherethrough to mass themselves and struggle abroad.
But contrariwise, when such a tenuous film
Of outside colour is thrown off, there's naught
Can rend it, since 'tis placed along the front
Ready to hand. Lastly those images
Which to our eyes in mirrors do appear,
In water, or in any shining surface,
Must be, since furnished with like look of things,
Fashioned from images of things sent out.
There are, then, tenuous effigies of forms,
Like unto them, which no one can divine
When taken singly, which do yet give back,
When by continued and recurrent discharge
Expelled, a picture from the mirrors' plane.
Nor otherwise, it seems, can they be kept
So well conserved that thus be given back
Figures so like each object.
Now then, learn
How tenuous is the nature of an image.
And in the first place, since primordials be
So far beneath our senses, and much less
E'en than those objects which begin to grow
Too small for eyes to note, learn now in few
How nice are the beginnings of all things-
That this, too, I may yet confirm in proof:
First, living creatures are sometimes so small
That even their third part can nowise be seen;
Judge, then, the size of any inward organ-
What of their sphered heart, their eyes, their limbs,
The skeleton?- How tiny thus they are!
And what besides of those first particles
Whence soul and mind must fashioned be?- Seest not
How nice and how minute? Besides, whatever
Exhales from out its body a sharp smell-
The nauseous absinth, or the panacea,
Strong southernwood, or bitter centaury-
If never so lightly with thy [fingers] twain
Perchance [thou touch] a one of them

Then why not rather know that images
Flit hither and thither, many, in many modes,
Bodiless and invisible?
But lest
Haply thou holdest that those images
Which come from objects are the sole that flit,
Others indeed there be of own accord
Begot, self-formed in earth's aery skies,
Which, moulded to innumerable shapes,
Are borne aloft, and, fluid as they are,
Cease not to change appearance and to turn
Into new outlines of all sorts of forms;
As we behold the clouds grow thick on high
And smirch the serene vision of the world,
Stroking the air with motions. For oft are seen
The giants' faces flying far along
And trailing a spread of shadow; and at times
The mighty mountains and mountain-sundered rocks
Going before and crossing on the sun,
Whereafter a monstrous beast dragging amain
And leading in the other thunderheads.
Now [hear] how easy and how swift they be
Engendered, and perpetually flow off