The truth is, that whenever a man is out of his mind, it is his soul that is demented—not because the mind is absent, but because it is a fellow-sufferer (with the soul) at the time. Indeed, it is the soul which is principally affected by casualties of such a kind. Whence is this fact confirmed? It is confirmed from the following consideration: that after the soul’s departure, the mind is no longer found in a man: it always follows the soul; nor does it at last remain behind it alone, after death.
Tertullian
A Treatise on the Soul
Chapter XVIII