Each episode of perception happens in two stages. For my purposes here, it is sufficient to say that in the first stage, an object external to us causes a disturbance to be conducted through our body (Ti. 43c1–3). In the second stage, the disturbance reaches the soul (43c4–d3). In the background of Plato’s account of perception is the view that each of the four so-called elements is made up of polyhedrons. Fire, for instance, is composed of tetrahedra. This informs how the body perceives things as hot or cold. The fire in something hot acts on our skin by cutting and dividing it, entering our body then (Ti. 61e–62a). We perceive sourness when the earth in what we eat is rough against our tongues; the less rough, the tangier the taste (65d). Colors are analyzed as flames that flow from objects (67c–d). Odors are more complicated: we cannot smell any of the elements. Plato thinks that our nostrils are too narrow for earth and water but are too wide to properly capture fire and air (66d). Instead, we perceive only the transitions between the elements.[5] Bodies produce odors when they decay, become damp, melt, or evaporate. In general, when there is some transitio

Plato does not talk about sense-data or sensory information being transmitted in these episodes. He talks instead of motions. For example, he speaks about more or less “penetrating” motions being produced by color-flames (68a–b). Sound is the percussion of air in the ear-canal, but hearing is the motion (κίνησις) that the percussion causes, which is transmitted from the head to the liver (67b–c).[6] These motions seem to be transmitted through the blood. When Plato explains how the rational kind of soul is disturbed by perception, he says that the violent motions join with the “perpetually-moving stream” (τοῦ ῥέοντος ἐνδελεχῶς ὀχετοῦ) in our body to reach and then stir the soul (43c–d). It might be at first be surprising, then, that the gods seemed to have designed the blood-stream exactly for this purpose: the gods connected the whole body with the veins so that no part of us was kept in the dark about what we perceive (77d–e).[7]

Perception exists as one cause of psychic disorders, alongside nutrition, bile, and phlegm. Yet, on the other hand, perception occupies a unique and perhaps unexpected status among these causes. For it seems that the gods deliberately created our bodies as capable of perception. This is not true of, say, nutrition: we need to nourish ourselves because the gods were incapable of furnishing us with a less needy body. In designing our bodies, the gods made certain concessions to necessity, but making us capable of perception does not seem to be one of them. For the gods deliberately designed our bodies with this in mind: consider that in the Timaeus, some perceptions are good for us. See the following passage:

The god invented sight and gave it to us in order that we might observe the revolutions of intelligence in the heavens and apply them to the revolutions of our own thought, since there is an affinity between them (Ti. 47b–c).

we must “correct the motions in our head that were corrupted at the time of our birth by learning the harmonies and revolutions of the cosmos” (90d). Perception is as useful for us as it is dangerous, but it is easy to miss this. Brisson (1997, 166), for instance, says “the contemplation of the universe and, above all, of the celestial movements is supposed to preserve the excellence of the soul. Otherwise, sensation may transform the soul into something bad.” He sets up a contrast between contemplation of the universe and sensation – but in such passages as 47b–c, the relevant kind of

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