For the most part of the Occidental history, that is how we have thought. Plato, for example, compared reason to a charioteer who must struggle to control two horses. One of them represents moral impulses, the “positive side” of our passionate nature. The other horse represents our irrational passions and appetites.
Spinoza was perhaps the first prominent thinker who noticed that instead of being antagonists, reason and emotions act together in order to make human culture and survival possible. A few centuries later, neuroscience would show that he was right. Moreover, decision making based on pure rational thinking is nearly impossible.
In the video below, Jonah Lehrer explains how our decision making process works.