For real existential-crisis-inducing marvels, you have to go with evolution. Once Darwin worked out natural selection, and it was combined with Mendel's genes, a whole host of primal questions were suddenly answered: Who created us? Why are we here? What is our purpose?
If you were to give Franklin a copy of "The Selfish Gene", I'm curious if he would believe it. Would he reject it and become (remain?) a creationist? Or would he accept that everything he believed about why we are here is wrong.
In fairness to the explicitly agnostic, I don’t think evolution or genetics are teleologies of humankind, or of consciousness, or even of simple life. These primal questions remain unanswered, if not unanswerable. Granted, a whole host of prior answers were revealed to be nonsense. (I’m sure Franklin would be unsurprised at this; an enlightenment polymath can hardly have credited the common man’s creationism - Jefferson, by way of comparison, was a rational deist.)
This is an excellent point. And you're probably right that Franklin and maybe Jefferson might have understood and accepted evolution--Darwin's world of the 1860s was not that different from Franklin's era.
But even today, I think many people do not understand the philosophical implications of evolution. Daniel Dennet wrote a whole book on this: "Darwin's Dangerous Idea" (which, perhaps you're familiar with). I think Franklin would have been shocked at evolution's explanatory power. It's hard to talk about things like emotions (anger, lust, jealousy), ethics (justice, retaliation), medicine, or even sexuality without invoking genetics and evolutionary theory.
I agree with you that evolution does not (and cannot) disprove the existence of God (or gods) or even that we were created 6,000 years ago (maybe God created the world to look as if it's ancient).
But I think Franklin would see that evolution has shrunk the numinous realm down to a very small box, and I think that would have shocked him.
Your response made me reconsider two of my claims. Evolution _can_ provide a teleology of both humankind and of consciousness. The latter may be understood as a genetic trait which provides extraordinary survival fitness, and so existence, to the former. We and our traits arise from, and have been fashioned to function within, the system of evolution. The traits, and us, want for no more purpose than this - just as the output of any function wants for no more purpose than the input parameters.
Life, though - that system and its output, holistically - still lacks self-evident purpose. (Which is not to say that it must have one.) Why was this function written? Is it merely the output of a higher-order function? (Involving cosmic turtles?)
If you were to give Franklin a copy of "The Selfish Gene", I'm curious if he would believe it. Would he reject it and become (remain?) a creationist? Or would he accept that everything he believed about why we are here is wrong.