Psych-ology is a Greek word meaning account (logos) of the soul (psyche). In this sense, the "soul" is whatever moves us by desire. But Plato observes that we are moved, sometimes in opposite directions, by three different sorts of desire: for wisdom, for status, and for money. There are basically three types of people according to which of these desires is strongest in the soul: wisdom-lovers (philo-sophers), status-lovers, and money-lovers. 

Recommended Readings:

Plato's Republic (Book 4, 436b7 - 441c2; Book 9, 588b9 - e2)

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