Plotinos: Complete Works, v. 4 In Chronological Order,...

In the soul weakness does not derive from an excess of density or rarefaction of leanness or stoutness, nor of any sickness such as fever. It must be met in souls which are either entirely separated from matter, or in those joined to matter, or in both simultaneously. Now, as it does not occur in souls separated from matter, which are entirely pure, and "winged,"193 and which, as perfect, carry out their functions without any obstacle; it remains, that this weakness occurs in fallen souls, which are neither pure nor purified. For them weakness consists not in the privation of anything, but in the presence of something alien, just as, for instance, weakness of the body consists in the presence of slime or bile. We shall, therefore, be able to understand clearly the weakness of the soul by ferreting out the cause of the "fall" of the soul.