In literature, reason is often opposed to emotions or feelings, and desires, drives or passions. Others see reason as the servant or tool of these things -- the means of sorting out our desires and then getting what we want. Some would say however that many of the key philosophers of history (e.g. Plato, Rousseau, Hume, Nietzsche) have combined both views - making rational thinking not only a tool of desires, but also something which is itself desired, not only because of its usefulness in satisfying other desires.

At the same time, reason sometimes clearly seems to come into conflict with some desires (even while not being in conflict with others) giving us the impression that reason is separate from emotion. Only in humans, choices are sometimes made on the basis of an association of ideas which is an artificially constructed model, rather than an un-inspected association based on raw experience, and this “feels” different from when one is won over by a passion supported by raw “feeling”. The opposite is also unique – we sometimes feel that a passion has won over our decision-making “unjustly”, despite having lost its argument, or perhaps (in the case, for example, of a reflex action) not even having been a subject of argument before the action took place.

The question of whether reason is in fact driven by emotions is important for philosophers because reason is seen by almost all philosophers as being the way that we come to know the truth, and we see the truth as something which exists outside of our own consciousness. If reason is driven by emotions, then how can we ever know that we are not deceiving ourselves about what is true by denying undesirable information in favor of a more pleasing construct of our world?

Fortunately, we are not entirely without answers to this important question. Modern psychology has much to say on the role of emotions in belief formation. Deeper philosophical questions about the relation between belief and reality are studied in the field of epistemology, which forms part of the philosophical basis of science, a branch of human activity that specifically aims to determine (certain types of) truth in an emotion-independent fashion.

Source : http://en.wikipedia.org