For Hume, there are no formal or final causes, just efficient and material, and even then he sheds considerable doubt on our abilities to determine induction and causality. Although he is part of the Enlightenment project as a whole, he nevertheless finds it necessary to denigrate reason to the status of slave to the passions. Its principle function is rationalization of actions undertaken, as a result of passions.
Actions cannot, according to Hume, be reasonable or unreasonable. They can only be laudable or unlaudable. So, he says, it’s not contrary to reason to prefer the destruction of the whole world to the scratching of his finger. This attitude is quite prevalent in the Treatise, although it softens up later in the Inquiry, for if taken absolutely literally, it would be self-contradictory. His exposition on the failure of reason requires a hefty dose of reason, so it is much like him sawing off the branch on which he is sitting.
In the Inquiry, he later acknowledges that reason may be a part of causing us to act, but the sentiments still are our prime motivation. His conception of morality is what Kant most complained about. Hume’s mostly engaging in philosophical anthropology. He’s demonstrating how humans do act, but not how they ought to act. But this criticism for Hume is irrelevant, since if there is no such thing as objective morality, then anthropology is sufficient. Any talk about duty is merely perspectival and illusory, which leads us to his main reasons for rejecting objective morality.
The first is the is/ought problem. Hume states that we cannot derive prescriptive statements from descriptive facts. It presents a challenge to the majority of moral systems, since they rely heavily on factual premises, in order to reach moral conclusions. But for Hume, this is nothing more than a deductive fallacy. This criticism alone, however, is not enough to totally defeat objectivity. He has to next introduce a newer epistemological division: relations of ideas and the facts of experience. Relations of ideas are statements that are trivially true; they’re tautologies, like ‘all unmarried men are bachelors.’ They say nothing new about the world, or anything particularly profound. On the other hand, we have facts of experience. Those are the fleeting, contingent observations of inductive reasoning, on which morality could never be based because of the is/ought problem.
So, according to Hume, since an objective account of morality cannot comfortable fit between the divisions, morality cannot be an expression of reason, but is instead based on the emotions and passions, which vary from culture to culture and from person to person. There is nothing objective in the terms ‘good’ and ‘evil’. They are nothing but an expression of personal approval or disapproval. A.J. Ayer and the other logical positivists later adopted this account wholesale, saying that when someone says murder is wrong, all they are saying is “boo murder!” Good actions, then, are those actions which tend to be preferred by people—like pleasure. And bad actions are the inverse: they cause pain.
This division of Hume’s is an attempt to improve upon Locke’s empiricism by importing Stoic and Epicurean (materialist) elements. He also has a large influence on Adam Smith’s work, and Hume already developed some fascinating economic theories of his own. For him, the most important feature of his philosophy is skepticism. He is suspicious of the claims of revealed religion, skeptical of induction and causality, skeptical of the meaningless postulates of Aristotle and the Scholastics, skeptical of attempts to create an objective morality, and skeptical of radical change at the hands of the societal rationalists. He aims to tear reason down from its lofty position and place it where it belongs, as slave to the passions and desires. We may not agree with him, but we can recognize the extent of his influence and appreciate subsequent attempts to answer him.