David Hume is an enigma—an enigma wrapped in a riddle, hidden in roll upon roll of good-natured corpulence. He is, first and foremost, an empiricist, someone who believes all knowledge is based on experience. What, after all, is more solid and trustworthy than experience? Show us an empiricist, and we expect to see an upstanding gentleman—a decent, respectable British fellow, a lot like Mr. Banks from Mary Poppins.
Well, it turns out that Hume doesn't quite fit the bill. He's Scottish, after all. Worse, he's a dude with some pretty wacky, unrespectable beliefs. Here's an empiricist who says we can't know that the sun will rise tomorrow, or that this pencil will actually fall the next time we drop it. He claims we can't be certain that a world outside of the ideas in our own mind actually exists.
And when it comes to literature? Sure, Hume pioneers the essay as a literary genre, and sure, he knows the great books through and through. But he says that literary taste, in the end, just rests on our sentiments or feelings—which is exactly what your stern high school teacher taught you never to say.
Okay, you say, so maybe this guy's just a skeptic, and he simply denies the possibility of knowledge altogether. Not exactly. Even in his very first book, A Treatise of Human Nature, Hume makes a major announcement: he's an experimenter. That means that when it comes to ideas, he'll trust experience and nothing else. If this sounds familiar, it's because our old buddy—and Hume's idol—Isaac Newton had used the same method. And Hume was sure that he could arrive at philosophical knowledge that was just as solid as Newton's.
Hume was so confident in his method that he actually thought he had identified the three "laws of association" that would explain how the mind works. (He calls these laws of association "resemblance," "contiguity," and "cause and effect.") That's right: Hume is sure he can tell you how your own mind works—and this was long before psychology was a thing.
In other writings, Hume applies this approach to literature. He says that our aesthetic judgments—like when we think a painting is pretty or a song moving or a poem...poetic...are rooted in those "feelings" that your high school English teacher warned you about. But that doesn't mean these judgments are purely subjective. Hume thinks that since every human has the same sentiments, this will allow us to discover the laws of aesthetics, just as Newton discovered the laws of physics.
But the greatest enigma of all is that Hume saw no contradiction between the scientific and skeptical strands of his thought. Contradiction, shmontradiction—it's all good. Hume thinks we can use empirical science to explain the physical world, literature, morality, and history—no problem. He just doesn't want us to build any big systems or philosophies based on these scientific explanations. In fact, he doesn't want us to build anything from these explanations. What you see is what you get, and that, says Hume, is what empiricism actually teaches us.
There have been empiricists before, of course, but none have been clear-minded enough, mischievous enough, or fearless enough to stay on that empiricist train even as it goes madly hurtling over the edge of rationality. Only David Hume had the mental girth to pull that off.