"What do you believe is true even though you cannot prove it?"

Leading scientists and creative thinkers were asked to address this question.  My favorite answer was from Judith Harris:

I believe, though I cannot prove it, that three - not two - selection processes were involved in human evolution.

The first two are familiar: natural selection, which selects for fitness, and sexual selection, which selects for sexiness.

The third process selects for beauty, but not sexual beauty - not adult beauty. The ones doing the selecting weren't potential mates: they were parents. Parental selection, I call it.

Nicholas Humphrey gave another good answer:

I believe that human consciousness is a conjuring trick, designed to fool us into thinking we are in the presence of an inexplicable mystery. Who is the conjuror and why is s/he doing it? The conjuror is natural selection, and the purpose has been to bolster human self-confidence and self-importance - so as to increase the value we each place on our own and others' lives.

The New York Times offers the full list of responses.  Arnold Kling offers further commentary.

Addendum: Eric Crampton points me to 120 additional answers, including Jared Diamond, Steven Pinker, and many other notables.  My answer?  I will go with Denis Dutton, with honorable mentions to Robert Trivers and Alexander Vilenkin.  I like Carlos Rovelli too.  Yes I know you don't all click on all of the links but these are worth checking out.