He claims to do so by appeal to "natural law," but as NMC observes, his arguments are unconvincing. This should surprise TBA readers so little that I will simply direct them to the article.
The article bears mention mainly because it points me to an instance of Hume's apparently contradicting himself. The article summarizes George's take on Hume:
Against Aristotle, Hume argued that the universe includes facts but not values. You cannot derive moral conclusions from studying the world, an “ought” from an “is.” There is no built-in, objective reason for me to choose one goal over another — the goals of Mother Teresa over the goals of Adolf Hitler, in George’s hypothetical. Reason, then, is merely a tool of whatever desire strikes my fancy. “Reason is and ought only to be the slave of the passions and may pretend to no office other than to serve and obey them,” George said, paraphrasing Hume, just as he does in seemingly every essay or lecture he writes.The "paraphrase" turns out to be a near-verbatim quote from the Treatise of Human Nature, 2.3.3.
The problem of course is that, if we can't derive an ought from an is, then Hume cannot say that reason *ought* to be the slave of the passions, merely because she is so in fact (according to him). I am a little surprised that this contradiction goes unremarked by the Nortons in their Oxford Philosophical Texts edition of the Treatise.
While the Treatise continues to have legions of students, I have always suspected that Hume was not without reasons for repudiating it in favor of the later Enquiries, and the foregoing contradiction -- like the fact that the "is/ought" argument does not appear in the Enquiries -- "ought" to suggest to students that Hume's repudiation should be taken more seriously than it is.
... Searching the text of the Enquiry Concerning the Principles of Morals for the "slave of the passions" quote, I don't find it repeated, but do stumble upon this gem of a note:
Solon's law forbids paederasty to slaves, as being an act of too great dignity for such mean persons.