Plato

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…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.