Full Idea
If the soul is composed of parts of the body, or the harmony of the elements composing the body, there will be many souls, and everywhere in the body.
Clarification
'Soul' is the Greek word 'psuché', which covers mind and consciousness and life
Gist of Idea
If the soul is composed of many physical parts, it can't be a true unity
Source
Aristotle (De Anima [c.329 BCE], 408a15)
Book Reference
Aristotle: 'De Anima (On the Soul)', ed/tr. Lawson-Tancred,H.C. [Penguin 1986], p.145
A Reaction
We will ignore "everywhere in the body", but the rest seems to me exactly right. The idea of the unity of the soul is an understandable and convenient assumption, but it leads to all sorts of confusion. A crowd remains unified if half its members leave.