Imagine you’re offered a delicious piece of cake. You know it’s very unhealthy. Your stomach tells you to eat it, but your brain tells you to refuse.
Maybe you listen to your brain and you ignore both the cake and your desire to eat it. You don’t follow your desire: instead you listen to reason.
Or, maybe your stomach wins out, and you eat the cake, but you feel like you made an irrational choice: desire wins out over what you know is the rational option.
Many think that reason and desire fight over things like the cake. We might say that if reason wins, we act rationally, and if desire wins, we act irrationally.
This common picture of human action is denied by Scottish philosopher David Hume (1711-1776). Hume instead claims that acting rationally means going along with our desires. That is a central idea expressed by his famous quote: “Reason is, and ought only to be the slave of the passions.” This essay explores Hume’s theory of how rationality and desire work together.
Hume uses “passion” as a technical term to describe things like pride and humility, desire and aversion, happiness and sadness, and fear. Broadly, passions are our emotions (e.g., feeling happy or sad), feelings (e.g., curiosity and confidence), and desires (e.g., wanting cake).
The passions are often contrasted with reason and rationality. In the cake example above, we imagined the brain and the stomach in a contest with each other. Hume agrees that the passions are in a different category from rational thought. But Hume thinks passions on their own are not rational or irrational. It is never irrational merely to feel pride, happiness, or any other passion. Feelings are reactions to our experiences. The cake looks good to you, so you want it: you don’t need to reason about it first. Nor do the passions and reason oppose each other. Instead, they work together.
2. Reason, Passion, and Action
Hume thinks that reason’s role is to help us act on our strongest passions.
According to Hume, reason is always playing a role in our actions. When you walk to the store to buy food, reason tells you which direction to walk, why you should bring money, and many other things. But Hume thinks that reason, on its own, never moves us to action.
Hume argues that unless you have a passion that leads you to walk to the store, you will never walk to the store. Reason alone can’t get you to buy food: you need a passion, like an aversion to hunger, or a desire to cook dinner to impress someone. Passion is in charge, and reason merely serves passion. “Reason,” he says, is “the slave of the passions.”
You might think Hume must be wrong, because we act against our passions all the time. If you leave the cake alone, isn’t that an example of reason winning out over passions?
Hume would reply that if you leave the cake alone, this is not a case of reason determining your action. Rather, your sudden, violent passion for the cake was beaten by a calmer, more settled passion for staying healthy.
So, Hume doesn’t think reason and passion are opposed to each other. Your passions determine what you will do, and you use reason to help you achieve those goals. You use reason to figure out how to best satisfy your strongest passions.
3. Irrational Actions
Because passions aren’t rational or irrational, the only things that can make actions irrational are false beliefs. For instance, it could be irrational to jump out of an airplane without a parachute if you falsely believe you can fly. But it wouldn’t be irrational to do this if you wanted more than anything to see if you could fly.
Indeed, according to Hume, any action, however odd it seems, can never be irrational if it’s driven by a passion that wins out over other passions. He even says it is “not contrary to reason to prefer the destruction of the whole world to the scratching of my finger.” Nor would it be irrational to allow your entire life to fall into ruin for the sake of making a complete stranger’s life slightly better.
Neither of these is irrational because neither passions nor actions based on them are irrational unless they are paired with false beliefs. If your aversion to having your finger scratched is strong and your desire to keep the world from being destroyed is weak, then it’s rational to let the world be destroyed rather than let your finger be scratched!
4. The Opposing View
In contrast to Hume, some philosophers think reason alone can sometimes tell us what to do. For instance, maybe it is always irrational to hurt yourself, or to hurt other people if they haven’t done anything wrong.
According to Hume, it would only be irrational to hurt yourself, or to hurt someone else, if your strongest passions oppose this. If someone doesn’t care about themselves, or they don’t care about other people, then according to Hume, they wouldn’t be acting irrationally if they hurt themselves or others.
Modern theories of action are more sophisticated than Hume’s. Later philosophers distinguished between two main questions addressed by Hume’s claim, both of which are still discussed today.
One question is about motivation: do we need passions to motivate us to act? If you don’t care even a tiny bit about your health or appearance, can you ever get yourself to exercise?
Another question is about reasons: does anyone ever have a reason to do something unless it is supported by a passion? If you don’t care at all about what happens to strangers, is there any reason for you to warn a stranger that they are about to fall off of a cliff?
But Hume’s central question of how reason and passions relate to each other is still relevant and passionately debated, and for good reasons.
 Hume, A Treatise of Human Nature 220.127.116.11.
 Hume, A Treatise of Human Nature 2. More examples: malice, envy, love, anger.
 Hume, A Treatise of Human Nature 18.104.22.168.
 Hume, A Treatise of Human Nature 22.214.171.124.
 Hume, A Treatise of Human Nature 2.3.3.
 Hume, A Treatise of Human Nature 126.96.36.199.
 Hume, A Treatise of Human Nature 188.8.131.52-10.
 Hume, A Treatise of Human Nature 184.108.40.206.
 Hume, A Treatise of Human Nature 220.127.116.11.
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