The following selection comes from the Second Book of De Rerum Natura. Here, Lucretius explains the motion of atoms and introduces his famous concept of "the swerve" (clinamen in Latin). According to Lucretius, all atoms are in motion all the time. Their natural tendency is to move down, because they have weight, but they can be propelled upward by external forces such as great gusts of wind. They mostly move up and down in straight lines, but "they swerve a little." These minute swerves are responsible for everything new that happens in the universe, as they introduce an element of unpredictability into an otherwise determined system. Nobody can predict what might happen when atoms swerve, and this uncertainty makes anything possible.
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This observation might seem simple, but its immense influence on modern scientific thought is the subject of The Swerve, a recent Pulitzer Prize-winning book by Harvard professor Stephen Greenblatt, which discusses the rediscovery of De Rerum Natura and its influence on the European Renaissance. According to Greenblatt, the discovery of the only surviving copy of Lucretius's poem in the early fifteenth century was itself a "swerve"—an imperceptible and unlikely change in fortune that had profound consequences for the world.
The selection excerpted here comes from Rolfe Humphries's 1968 translation titled The Way Things Are. Unlike the many modern prose translations of De Rerum Natura, Humphries preserves Lucretius's original intention to work in poetry, which establishes scientific arguments with both the force of its ideas and the beauty of its language.
If you think
Atoms can stop their course, refrain from movement, And by cessation cause new kinds of motion, You are far astray indeed. Since there is void
Through which they move, all fundamental motes[135] 5
Must be impelled, either by their own weight
Or by some force outside them. When they strike
Each other, they bounce off; no wonder, either,
Since they are absolute solid, all compact,
With nothing back of them to block their path. 10
To help you see more clearly that all atoms
Are always moving, just remember this:
There is no bottom to the universe,
No place for basic particles to rest,
Since space is infinite, unlimited, 15
Reaching beyond all bounds, in all directions,
As time and time again I have shown you proof.
Therefore, of course, no atom ever rests
Coming through void, but always drives, is driven
In various ways, and their collisions cause, 20
As the case may be, greater or less rebound. When they are held in thickest combination, At closer intervals, with the space between More hindered by their interlock of figure, These give us rock, or adamant[136], or iron,
Things of that nature, (Not very many kinds Go wandering little and lonely through the void.) There are some whose alternate meetings, partings, are At greater intervals; from these we are given
Thin air, the shining sunlight. Many more 30
Have been kept out of any combination,
Nowhere conjoin. Before our eyes we have
An illustration. If you look sometimes,
You see the motes all dancing, as the sun
Streams through the shutters into a dark room. 35
Look!—there they go, like armies in maneuver
Whose little squadrons charge, retreat, join, part,
From this you can deduce that on a scale
Oh, infinitely smaller, beyond your sight,
Similar turbulence whirls. A little thing 40
Can often show us what a great one's like,
And that's not all the story, either. Watch!—
Those motes in the sunlight, by their restlessness,
Tell you there's motion, hidden and unseen,
In what seems solid matter. As they bounce, 45
Change course, come back, here, there, and every which way,
You may be sure this restlessness is given
By their essential core, atomic essence,
From just these first-beginnings. They are moved
By their own inner impulse first, and then 50
Such groups as form with just a few together,
Only a little bigger than their units,
Are moved by unseen blows from these: in turn
They shove along the somewhat larger masses.
So, motion comes from first-beginnings, grows 55
By slow degrees till we can see the process,
Just as we see the dancing motes in the sunlight,
But cannot see what urge compels the dancing.
Now then—what kind of speed does matter have?
The answer, Memmius,3 won't take very long. 60
When dawn bathes earth with morning light, and birds,
All kinds of them, flying through pathless woods,
Fill all the delicate air with liquid song,
How suddenly at such a time the sun
3. Memmius: Gaius Memmius, a Roman poet and statesman to whom Lucretius dedicated De Rerum Natura.
Clothes everything with light! This we can see, 65
And so can all men, plain before their eyes.
But the sun's warmth and that calm light come on
Not through an empty void; their course is set
More slowly, as if they parted waves of air
The way a swimmer does. Not one by one 70
Do the tiny particles of heat proceed,
But rather en masse, get in each other's way,
At times, and are also blocked by outside force.
All this combines to make them go more slowly.
It does not work this way with single atoms 75
Which go along through empty void, unchecked
By opposition. They have their parts, of course,
But they are single units; they drive on,
Resistless, toward their first direction's impulse,
And they must be of marvelous speed, beyond 80
The speed of light, surpassing far the sweep
Of lightning in split seconds through the sky—
Impossible to follow every atom,
To see complete their order, action, system.
Some people do not know how matter works. 85
They think that nature needs the will of the gods
To fit the seasons of the year so nicely
To human needs, to bring to birth the crops
And other blessings, which our guide to life,
The radiance of pleasure, makes us crave 90
Through Venus'[137] agency. To be sure, we breed To keep the race alive, but to think that gods Have organized all things for the sake of men Is nothing but a lot of foolishness.
I might not know a thing about the atoms, 95
But this much I can say, from what I see Of heaven's ways and many other features: The nature of the world just could not be A product of the gods' devising; no,
There are too many things the matter with it. 100
I'll give you further details, Memmius, later. Now to get on with further explanation Concerning motion.
The first point to make Is, no internal force can make things rise 105
Or force them upward. Don't be fooled by sparks: I know they rise, increase, grow upward; crops Act the same way, and trees, although their weight Exerts a downward pull. We must not think,
When fires go leaping toward the roofs of houses, 110
And the swift flames lick out at beams and timbers,
That they do this of their own will entirely,
Without the urge of pressure from below.
Blood acts the same way, spurting from our bodies,
Arterial jets, a scarlet-colored fountain, 115
And don't you see how violently water
Geysers beams and timbers? The deeper we shove them,
The harder we push them down, all might and main—
With just that energy water shoots them back,
Heaving them up till they leap clear into air. 120
Yet, I suppose, we have no doubt all things,
So far as in them lies, are carried down
Through empty space. So flames, through wafts of air,
Must rise, impelled, although their weight fights back
To bring them downward. Don't you see the torches 125
Of the night sky draw their long fiery trails