Plato quotes - page 13
Some say that the body (σῶμα) is the "tomb" (σῆμα) of the soul, their notion being that the soul is buried in the present life; and again, because by its means the soul gives any signs which it gives, it is for this reason also properly called "sign" (σῆμα). But I think it most likely that the Orphic poets gave this name, with the idea that the soul is undergoing punishment for something; they think it has the body as an enclosure to keep it safe, like a prison, and this is, as the name itself denotes, the "safe" (σῶμα) for the soul, until the penalty is paid, and not even a letter needs to be changed.
Plato