Humeschwitzstein

Astronomers know -- or seem to know -- the most remarkable things on the slenderest base of evidence. Largely on the basis of the spectral analysis of light (often supplemented with verbal reports about rare events in very small pieces of the world called physics laboratories), they reach conclusions about the origin and early history of the universe, about unfamiliar events at virtually unimaginable distances, about the destiny of the sun in some billions of years. It could, of course, all have been one giant projection screen overhead that we'd have punctured with our first moon probe.

Why assume that as things happen here, they happen everywhere? -- that as things happen now, they've happened always and will happen forever? Hume compared us (didn't he?) to fleas on the back of a dog, watching a hair grow and saying "ah, that's how the universe works!"

Some days, I feel very much like such a flea. Hume used this idea, or something like it, in defense of atheism, or something like it; but in me it works the other way. Our science does a pretty good job with our local little hair, Earth, and the nearby hairs (Mars, etc.) seem to grow under much the same rules; but should I be so confident there isn't a whole dog somewhere underneath whose operation I haven't even begun to imagine?

Nor can we even remember or keep straight the growth of our little hair. I search for the Hume quote. I can't find it in his Dialogues Concerning Natural Religion, which would have been my guess. I have a sense that I heard my colleague Howie Wettstein attributing the quote to him. So I email Howie and he tells me that the part about the flea on the dog is pure invention on my part but the back half of the quote is accurate; but now he can't find it in Hume either.

Finally we trace it to the source: Dialogues Concerning Natural Religion, Part II:

From observing the growth of a hair, can we learn any thing concerning the generation of a man?

(In context, Hume is challenging the assumption implicit in the argument from design that we can transfer what we know about the workings of small, local things to the workings of universe as a whole.)

Howie remarks that it's like that old game "telephone": Hume said it, Howie misremembered it to me, I misremembered it to my students and to you just now. All the more reason for humility, Hume might say.