Full Idea
Since the substance of the mind is extraordinarily mobile, it must consist of particles exceptionally small and smooth and round, ..so that, when the spirit has escaped from the body, the outside of the limbs appears intact and there is no loss of weight.
Gist of Idea
The mind is very small smooth particles, which evaporate at death
Source
Lucretius (On the Nature of the Universe [c.60 BCE], III.201)
Book Reference
Lucretius: 'On the Nature of the Universe', ed/tr. Latham,Ronald [Penguin 1951], p.102
A Reaction
Lucretius is wonderfully attentive to interesting evidence. He goes on to compare it to the evaporation of perfume. The fine-grained connections of the brain are not far off what he is proposing.