What does David Hume mean when he states that reason alone cannot be a motive to the will, but rather it is the slave of the passions in Treatise on Human Nature?

David Hume states that reason is and should be the slave of passions in Treatise on Human Nature, and by this he means that what is considered "reasonable" is subjective and ultimately relies on one's passions to determine. Furthermore, reason can't decide one's passions, but once a passion has been decided reason can help develop it.

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David Hume wrote in his Treatise on Human Nature:

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.

The phrasing here is colorful, but what Hume has to say is itself entirely reasonable. Reason cannot provide any motivation to do anything. At the end of every chain of reasoning, there is a simple assertion. We might, for example, assert that a certain political decision is right on the Utilitarian grounds that it provides the greatest happiness for the greatest number of people. Why is that good? Because happiness is a good thing. Why is it a good thing? Every parent who has experienced a conversation with a child asking "Why?" in response to every statement knows that sooner or later the answer is: "Because I say so."

If someone will not admit that happiness is a good thing, there is no way to prove that it is using reason alone. This is why Hume asserts not only that reason is the slave of the passions, but that it ought to be. Once you have an objective, reason can help you to attain it, but it cannot provide you with the objective. This point is often made in discussions about Artificial Intelligence. If we assume that Artificial Intelligence does not include artificial desire or artificial emotion, then the principal objection to a scenario such as the one portrayed in The Matrix, in which machines take over the earth, is the point Hume makes here. Humans want power in the same way that we need food, because we are animals. A machine which is purely logical would have no more reason to want power (or anything else) than it would to need food.

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