Sagan's letter to Leibniz

From Ann Druyan's introduction to The Varieties of Scientific Experience by Carl Sagan.

In [the] same drawer where the transcript of these lectures were rediscovered, there was a sheaf of notes intended for a book we never had the chance to write. Its working title was Ethos, and it would have been our attempt to synthesize the spiritual perspectives we derived from the revelations of science. We collected filing cabinets' worth of notes and references on the subject. Among them was a quotation Carl had excerpted from Gottfried Leibniz (1646 - 1716), the mathematical and philosophical genius, who had invented differential and integral calculus independently of Isaac Newton. Leibniz argued that God should be the wall that stopped all further questioning, as he famously wrote in this passage from Principles of Nature and Grace:

"Why does something exist rather than nothing? For 'nothing' is simpler than 'something.' Now this sufficient reason for the existence of the universe ... which has no need of any other reason ... must be a necessary being, else we should not have sufficient reason with which we could stop."

And just beneath the typed quote, three small handwritten words in red pen, a message from Carl to Leibniz and to us: "So don't stop."