Full Idea
Cleanthes says no incorporeal interacts with a body, but one body interacts with another body; the soul interacts with the body when it is sick and being cut, and the body feels shame and fear, and turns red or pale, so the soul is a body.
Gist of Idea
Bodies interact with other bodies, and cuts cause pain, and shame causes blushing, so the soul is a body
Source
report of Cleanthes (fragments/reports [c.270 BCE]) by Nemesius - De Natura Hominis 78,7
Book Reference
'The Hellenistic Philosophers:Vol.1 translations', ed/tr. Long,A. /Sedley,D. [CUP 1987], p.272
A Reaction
This is precisely the interaction problem with dualism, or, as we might now say, the problem of mental causation. The standard Stoic view is that the soul is a sort of rarefied fire, which disperses at death.