Full Idea
Aristotle insists [against Plato] that desires, even rational desires, are a capacity distinct from reason, as is perception. Belief is included within reason. And he sometimes distinguishes steps of reasoning from insight.
Gist of Idea
Aristotle makes belief a part of reason, but sees desires as separate
Source
report of Aristotle (De Anima [c.329 BCE], 428-432) by Richard Sorabji - Rationality 'Shifting'
Book Reference
'Rationality in Greek Thought', ed/tr. Frede,M /Striker,G [OUP 1999], p.330
A Reaction
So the standard picture of desire as permanently in conflict with reason comes from Aristotle. Maybe Plato is right on that one (though he doesn't say much about it). Since objectivity needs knowledge, reason does need belief.