On the nature of things (books 1 - 3)   

And whence does such a mass of living things,
Boneless and bloodless, o'er the bloated frame
Bubble and swarm? But if perchance thou thinkest
That souls from outward into worms can wind,
And each into a separate body come,
And reckonest not why many thousand souls
Collect where only one has gone away,
Here is a point, in sooth, that seems to need
Inquiry and a putting to the test:
Whether the souls go on a hunt for seeds
Of worms wherewith to build their dwelling places,
Or enter bodies ready-made, as 'twere.
But why themselves they thus should do and toil
'Tis hard to say, since, being free of body,
They flit around, harassed by no disease,
Nor cold nor famine; for the body labours
By more of kinship to these flaws of life,
And mind by contact with that body suffers
So many ills. But grant it be for them
However useful to construct a body
To which to enter in, 'tis plain they can't.
Then, souls for self no frames nor bodies make,
Nor is there how they once might enter in
To bodies ready-made- for they cannot
Be nicely interwoven with the same,
And there'll be formed no interplay of sense
Common to each.
Again, why is't there goes
Impetuous rage with lion's breed morose,
And cunning with foxes, and to deer why given
The ancestral fear and tendency to flee,
And why in short do all the rest of traits
Engender from the very start of life
In the members and mentality, if not
Because one certain power of mind that came
From its own seed and breed waxes the same
Along with all the body? But were mind
Immortal, were it wont to change its bodies,
How topsy-turvy would earth's creatures act!
The Hyrcan hound would flee the onset oft
Of antlered stag, the scurrying hawk would quake
Along the winds of air at the coming dove,
And men would dote, and savage beasts be wise;
For false the reasoning of those that say
Immortal mind is changed by change of body-
For what is changed dissolves, and therefore dies.
For parts are re-disposed and leave their order;
Wherefore they must be also capable
Of dissolution through the frame at last,
That they along with body perish all.
But should some say that always souls of men
Go into human bodies, I will ask:
How can a wise become a dullard soul?
And why is never a child's a prudent soul?
And the mare's filly why not trained so well
As sturdy strength of steed? We may be sure
They'll take their refuge in the thought that mind
Becomes a weakling in a weakling frame.
Yet be this so, 'tis needful to confess
The soul but mortal, since, so altered now
Throughout the frame, it loses the life and sense
It had before. Or how can mind wax strong
Co-equally with body and attain
The craved flower of life, unless it be