On the nature of things (books 1 - 3)   

Thou must admit, besides all blows and weight,
Some other cause of motion, whence derives
This power in us inborn, of some free act.-
Since naught from nothing can become, we see.
For weight prevents all things should come to pass
Through blows, as 'twere, by some external force;
But that man's mind itself in all it does
Hath not a fixed necessity within,
Nor is not, like a conquered thing, compelled
To bear and suffer,- this state comes to man
From that slight swervement of the elements
In no fixed line of space, in no fixed time.
Nor ever was the stock of stuff more crammed,
Nor ever, again, sundered by bigger gaps:
For naught gives increase and naught takes away;
On which account, just as they move to-day,
The elemental bodies moved of old
And shall the same hereafter evermore.
And what was wont to be begot of old
Shall be begotten under selfsame terms
And grow and thrive in power, so far as given
To each by Nature's changeless, old decrees.
The sum of things there is no power can change,
For naught exists outside, to which can flee
Out of the world matter of any kind,
Nor forth from which a fresh supply can spring,
Break in upon the founded world, and change
Whole nature of things, and turn their motions about.

ATOMIC FORMS AND THEIR
COMBINATIONS

Now come, and next hereafter apprehend
What sorts, how vastly different in form,
How varied in multitudinous shapes they are-
These old beginnings of the universe;
Not in the sense that only few are furnished
With one like form, but rather not at all
In general have they likeness each with each,
No marvel: since the stock of them's so great
That there's no end (as I have taught) nor sum,
They must indeed not one and all be marked
By equal outline and by shape the same.

Moreover, humankind, and the mute flocks
Of scaly creatures swimming in the streams,
And joyous herds around, and all the wild,
And all the breeds of birds- both those that teem
In gladsome regions of the water-haunts,
About the river-banks and springs and pools,
And those that throng, flitting from tree to tree,
Through trackless woods- Go, take which one thou wilt,
In any kind: thou wilt discover still
Each from the other still unlike in shape.
Nor in no other wise could offspring know
Mother, nor mother offspring- which we see
They yet can do, distinguished one from other,
No less than human beings, by clear signs.
Thus oft before fair temples of the gods,
Beside the incense-burning altars slain,
Drops down the yearling calf, from out its breast
Breathing warm streams of blood; the orphaned mother,
Ranging meanwhile green woodland pastures round,
Knows well the footprints, pressed by cloven hoofs,