Philosopher David Hume
wrote this famous quote, "reason is, and ought only to be the slave of the passions," in his 18th century
work, A Treatise of Human Nature
, (18.104.22.168). Making sense of this is tough, since usually we think that we can reason ourselves into being less emotional about things. Hume says that this is wrong, as I'll explain below.
Hume claims that the passions make no reference to external things, and that they are original existences, because their domain is exclusively the mind. In a way, he seems to be saying that the passions are the very substance of the self, (but that's a big simplification, I'm sure).
The claim that the passions don't refer to external things may at first seem ludicrous; after all, if someone punches me in the face, is my ensuing anger not a reference to the punching? Or the fist? The answer, perhaps surprisingly, is no; the passion (anger) that follows a punch to my face does not make a reference to any external object or occurrence, although it is undeniably associated with that punch. I do form thoughts referring to the assault, but these thoughts are devoid of any passion; they are mere compilations of data, combining sensory perception with extant knowledge. "I have been punched. My jaw hurts. Someone has assaulted me. Assault is illegal." My passions are ignited in response to those thoughts, but the only 'referent' of my anger, if we must identify one, is my inner self. Not the fist, not the punch, and not even my thoughts.
The above example could tempt me to think that by using persuasive reasoning with myself, I could control my behavior with rational thoughts, despite my anger. For example, I might think to myself, "I have the option of restraining this man or hitting back. Hitting back could result in my arrest, while restraining him will probably not. I ought to restrain him, and avoid getting myself arrested." But notice this: while the first two sentences are statements of fact, the last one is a statement of desire. The "ought" statement seems to be derived from the "is" statements, but it does not logically follow from them. (Why ought one avoid arrest?) The ingredient of "ought-ness" is a sly insertion of the passions into the reasoning process. There is an 'unstated' thought, left out above: "I ought to do the thing which best serves my passions--my desires." This is not reason coaching the passions—-it's the reverse! This is what Hume means when he writes that reason is a slave of the passions. Reason's role in guiding actions is limited to its utility in aiding the fulfillment of desire--in responding to the passions--of the self.