What did David Hume mean when he said that "reason is a slave to the passions"?
I don't understand the meaning of this oft-quoted quotation of Hume's in On Reason, namely his saying that "reason is a slave to the passions." What exactly does he mean by that ? Is it simply that reason is subsequent to a deeper moral sense? Is it equivalent to the maxim today that "science cannot answer moral questions"? One thing that may be confusing is me is that I sense he's being somewhat rhetorical; would it be better to summarize his the arc of work that reason can only guide the passions, and that the truths we think it is uncovering us are ultimately a product of what our fickle passions urge it to investigate?
You can take it almost literally. Reason while independent and pure in its functioning is given tasks not by itself but by the human passions. Existentialism is for instance the most reasonable philosophy but (as a Not) does not command man (and in turn the tasks for reason) to anything whatsoever. it is the response to the nihilism which moves man.
Hume's quotation is from a famous passage discussing the "motivating influence of the will" in his Treatise on Human Nature and reads in full:
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. (T 2.3.3 p. 415)
The context is his discussion of what is sometimes called "moral psychology", the study of how we are motivated to act morally. In particular, he raises a question about the role of practical reason in moral motivation. Hume vehemently opposes the view, held by philosophers before him (and after him), that to act morally is have a rational grasp of moral truths. He defends an instrumental conception of practical reason, according to which the role of reason is only to find out which means helps achieve a given goal. Reason (or the intellect) plays no part in determining the goals. Our goals are set exclusively by what Hume calls the passions and what today is most often called desires. Desires cannot be evaluated as true or false or as reasonable or unreasonable - they are "original existences" in our mind and arise from unknown natural causes. We cannot be criticized rationally for our desires (As Hume remarks, it is "not contrary to reason to prefer the destruction of the whole world to the scratching of my finger" (p 416)).
Reason is the slave of the passions in the sense that practical reason alone cannot give rise to moral motivation; it is altogether dependent on pre-existing desires that furnish motivational force. For Hume, this is not a fact we should lament (as moralists do) but a basic fact about our psychology.