Plato

Plato’s theory of the soul is found in The Republic, where it is part of a response to the Sophists as to why one ought to live morally. The Sophists in Plato’s time were men who used philosophy for profit. For a fee they would teach others the art of negotiation and argument. They also would create moral loopholes to excuse what would otherwise be immoral behavior. The skeptics ask Plato why one …

…soul will become unhappy if one of its parts is overindulged. For example, if a particular appetite were overindulged, then one would become fat, or obsessed, or drunk, or some other unhappy state depending on which appetite was overindulged. As noted above, if the spirit or reason is overindulged, other unhappy conditions will come as a result. So the final way to know when a soul is just is to see if it is happy.