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?
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.
You can apply this quotation in many different contexts as far as Hume's thought is concerned - in general I think the best way to read it is as an outgrowth of his radical empiricism which in the case of ethics descends into his famous advocacy of emotivism. The point is that reason will never reach out into the world - the passions are what we get when the world reaches into us. And therefore, reason will never be able to control them or understand them because they (the passions/sensations/impressions) are the raw materials of reason. So here are some ways to understand this general tendency, as expressed in the famous maxim you quote:
- Emotivism in Ethics: Reason cannot enter into our ethical judgements because these judgements are based on sentiments (i.e. passions.) An act of cruelty will cause in us a feeling of injustice, and that feeling (sentiment/passion) will be the reason why we pass an unfavourable judgement on an act of cruelty. This is the polar opposite of a Kantian view of ethical judgements to which one arrives at by pure a priori reasoning.
- Taste in Aesthetics: Here the situation is more complicated. Hume admits that beauty can only ever be as it were 'in the eye of the beholder' because beauty cannot be in the object but must be wholly contained in the pleasurable sentiment it causes us. Indeed Hume explicitly states that pleasure is the essence of beauty (we define as beautiful that which gives us pleasurable sensations.) So here you see that reason will always be a slave of the passions, i.e. you will never be able to rationally convince your friend who thinks artworks by Dali are ugly that they are, in fact, beautiful. Nevertheless in On the Standard of Taste Hume tries to argue that there are in fact some objective aesthetic standards, by urging us to heed the advice of ideal critics which he goes on to define (i.e. critics possessed of a delicacy of taste, and sound understanding, sharpened by practice and comparison and who are free from prejudice.)
- Self and Causation: Hume also famously argued that there is no such thing as direct causation, only observable regularities; and that there is no indestructible self, only impressions which we call our own. If you take passions to include sensations of hot or cold or sense impressions, then you can interpret the maxim that reason will always be a slave to such passions as a further advocacy of this type of empiricism or anti-realism. The latter (anti-realism) in the sense that there are no shared constraints that ever effectively decide such questions as 'Am I here?' or 'Is this colder than this?'
Hume's argument is that all preferences and motives are emotional. There is no such thing as an unemotional or purely rational decision, because to decide, by its nature, is to have a preference for, i.e. an inclination toward or aversion to, something.
Reason's (i.e., cognition's) role is to structure the world for us: it lays out a schematic of how objects and ideas are connected. But this is merely cold information, devoid of any significance on its own. Rational categories have no value or priority without feeling.
The passions (i.e., emotions or affects) are necessary to evaluate any object or idea as valuable, problematic, virtuous, immoral, good, or bad, because these forms of evaluation are all based on either a positive or negative impression. Decisionmaking is neither an rational nor "irrational" (whatever the hell that means) process: it is based on preferences, which must arise from emotional states and can never just magically be produced by reason alone.
What reason does is enable things to become positive or negative by association. I have a goal that I care about, and reason suggests to me that a previously unimportant object will help me achieve it. Thus the object itself becomes important to me. Reason creates the association--it's a telescope that allows me to see the distant or indirect emotional consequences of the object--but the emotion itself remains my only motive and only decider.
In short: there are beliefs and desires. What Hume says (and that's also what is true) is that only desires can make us act / give reasons / determine our goals and the purpose of us / tell us what is good or right / determine what is moral. Beliefs and rationality (reason) are just an instrument, a slave, for achieving the goals given by the desires. (of course trough this function reason can set instrumental goals, like telling us that we should not jump from the bridge if we want to live, but in the end it is still the desire to live which keeps us from jumping (or not, if we want to die)).
contrary to what some said here: this has nothing to do with empiricism, which only concerns the kind of sources that beliefs can be founded on.
To use an analogy, a map by itself does not tell you where to go, only how to get there once you have decided. Hume is saying that reason, or logic, cannot tell how you how to act or what to choose. It is a "slave" to your "passion" similar to how a map is a slave of choosing where you want to go. This is in contrast to Kant, who believed that reason can tell you how to act morally.
To understand the quotation, one needs to understand what is meant by reason and passion. It is commonly understood that "reasonable" persons think before acting, whereas "passionate" (emotional) people, act without thinking. So, "reason" is associated with logical, methodical, reasonable. "passion" is associated with emotional, illogical, unreasonable. Most logical (reasonable) persons see themselves as having control of their emotions, whereas passionate people are thought of as being controlled by their emotions. What David Hume is saying, is that those that think they have control of their emotions, are only fooling themselves, because they are "rational" only to the extent that their emotions allow them to be. An example of this, is love. If you are in love, reason "goes out the window." So does sight, reasonableness, thought, and logic!
This statement arises from Hume's definition of the intellect, he never believed the intellect to offer more than it took in i.e. the job of the intellect is to deal with what the senses can perceive, once information is stored and events experienced the intellect has a role to play
The role of the intellect is therefore restricted to the physical and has no relevance to the metaphysical, we cannot therefore, according to Hume, accept anything beyond matter to be intelligible by the intellect
It is the philosophy of today's materialism,and since passions play a role in determining what materials should be selected and used by a human, the intellect is subservient to it
Immanuel Kant did not differ too much in his definition of the intellect and his works could help you if you want to understand Hume