The Republic - pg. 261

"One part, we say, we say, was that with which a human
being learns, and another that with which he becomes
spirited; as for the third, because of its many forms, we
had no peculiar name to call it by, but we named it by what
was biggest and strongest in it. For we called it the desiring
part on account of the intensity of the desires concerns with
eating, drinking, sex, and all their followers; and so, we also
called it the money-loving part, because, such desires are
most fulfilled by means of money."