“Reason is, and ought only to be, the slave of the passions, and can never pretend to any other office than to serve and obey them,” Scottish philosopher David Hume famously said in his Treatise of Human Nature. He also offered a theory about the way that emotions spread among people, which helps explain the divisiveness of our current politics.

How Actions are Produced

According to Hume, all our intentional actions are the immediate product of passions -- or emotions, feelings, or desires, as we would say today. He does not think that any other kind of mental state could, on its own, bring about an intentional action, except if it first generates a passion. Not all the passions can become motives for action, however. Beyond instincts such as hunger and lust, only desire and aversion, hope and fear, joy and grief, or combinations of these, generate action. How? A feeling of pleasure or pain, whether physical or psychological, or a belief that pleasure or pain may or will come from something, occurs in the mind and causes the passion that brings about action.