Sunday Science Poem: The Epicurean theory of vision, plus bedwetting in ancient Rome

640px-Ercolano03For those of you looking for a break from the pre-game show, here’s this week’s reading of The Nature of Things.

Discard, for the moment, everything you know about vision – about light sources that emit photons of various frequencies, about photons that pass through or reflect from materials before impinging on the photoreceptor cells in your retina, stimulating your optic nerve, and generating an image in your brain. As we set aside our modern knowledge and begin from scratch to think about the surprisingly knotty problem of how visual images are faithfully transmitted and perceived across the distance between an object and our eyes, it’s not hard to understand why Lucretius’ Epicurean theory of vision is so convoluted and not very explanatory.

Lucretius didn’t know anything about scattered and reflected photons, but he deeply believed that the world could be explained by the action of particles. So his theory of vision in Book IV of The Nature of Things is based on objects emitting particles:

Let me begin
by saying there are images of things - a sort of skin
shed from the surfaces of objects, from the outer layer -
films that drift about this way and that upon the air…”
- Book IV, lines 29-32

This theory of thin films constantly shed by all objects explains why the red, gold, and purple fabric of the awnings at Roman theaters cast a colored hue on the audience:

For clearly we see many things emit bodies galore,
Not just from deep inside themselves, as I have said before,
But also from exteriors, for instance with their hue - 
As red and gold and purple awnings regularly do,
When, stretched over a great theatre, hung on poles and beams
Above the crowd, they flap and billow, and their colour streams,
Staining the faces of the audience in the stands below,
The stage and spectacle a-flicker in their fluttering glow.
The more the theater’s enclosed, shut from the glare of the day,
The more the laughing show of colours puts on its display.
Therefore, since linen, from its outer surface, gives a hue,
So everything must be emitting flimsy images too - 
Since, as with colour, these are thrown off from the outer surface.
There are such things as films, therefore, that keep the certain trace
Of forms, flying everywhere, of such a gauzy weave
That separately, one by one, they’re impossible to perceive.

- Book IV, lines 72-89

Lucretius uses this theory to explain mirror images, distortion of shapes over distances, and shadows (which are caused by gloomy air that physically blocks the films emitted by brighter things). He even uses his theory to explain purely mental images – just as clouds can spontaneously mimic faces, animals, and other terrestrial objects, drifting films will spontaneously, in their interaction with our minds, form our dreams. Reading through the details of the Epicurean theory of vision, it’s easy for us to spot holes in the ideas, but after the little thought experiment I proposed above, you can quickly come to appreciate how difficult it is to explain visual perception if you’re starting from no prior knowledge.

More impressive are the beautifully described details of first century BCE Roman life that Lucretius uses to illustrate his ideas. The colors of Roman awnings that cast purple shadows on the spectators, the odors of Roman spices, Roman children spinning in circles between stone columns to make themselves dizzy, or wetting the bed and their fine Babylonian coverlets, or later, having wet dreams, these details make daily life of a now mythic civilization seem close and familiar.

The vivid images of life in Book IV come after a meditation on death and the mortality of the soul in Book III. Lucretius makes the core Epicurean argument that, because all things are made of atoms, even our souls, there is no afterlife. Sound familiar? We’re still having the same conversation 2000 years later, even though our scientific understanding has a more thoroughly developed materialistic basis.

Because the universe is made only of atoms, our souls are mortal. Because our souls are mortal, because our time is limited, and because we have no need to fear eternal punishment, Lucretius argues we should be focused on making this life better. When we fear an afterlife that doesn’t exist, we’re weighed down with unnecessary cares, and we’re not living the lives we would choose to live if we knew the truth.

And Cerberus, and the Furies, and the dearth of light as well,
And noxious vapours belching forth out of the maws of Hell - 
These do not anywhere exist, nor can they, it is clear.
Rather, it’s punishment in life for misdeeds that we fear,
In proportion to the dreadful deed, and how we must atone
For crimes committed: locked away in a prison cell, or thrown
Traitor’s Rock; the cat-o’-nine-tails, torturers, the rack,
Pitch, red-hot metal plates, firebrands - for even though we lack
Such things, the Conscience fears for its misdeeds, and it applies
Goads and floggings to itself, neither does it surmise,
Meanwhile, that there can be, to all its sufferings, an end,
Nor, at length, a final limit to its punishment,
So it dreads these same oppressions will grow weightier with death, 
Until, at last, the life of fools becomes a Hell on Earth.

- Book III, lines 1011-1023

Last week’s reading: Lucretius did not believe in non-overlapping magisteria.

All quotations are from the translation by A.E. Stallings in the Penguin edition (2007)
Image credit: Tim Brighton, House of Neptune and Amphitrite, Herculaneum, via Wikimedia Commons.

Author: Mike White

Genomes, Books, and Science Fiction

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