  The Republic offers two general reasons for the  tripartition. First, Socrates argues that we cannot coherently  explain certain cases of psychological conflict unless we suppose  that there are at least two parts to the soul. The core of this  argument is what we might call the principle of non-opposition: “the  same thing will not be willing to do or undergo opposites in the same respect, in relation to the same thing, at the same time” (436b8–9).  This is a perfectly general metaphysical principle, comparable to  Aristotle’s principle of non-contradiction (Metaphysics G3  1005b19–20). Because of this principle, Socrates insists that one  soul cannot be the subject of opposing attitudes unless one  of three conditions is met. One soul can be the subject of  opposing attitudes if the attitudes oppose each other at different times,  even in rapidly alternating succession (as Hobbes explains mental  conflict). One soul can also be the subject of opposing attitudes if  the attitudes relate to different things, as a desire to drink  champagne and a desire to drink a martini might conflict. Last, one  soul can be the subject of opposing attitudes if the attitudes oppose in different respects.