There is in the lovely Martian landscape not a footprint, not an artifact, not even an old beer can, not a blade of grass, not a kangaroo rat, not even, so far as we can tell, a microbe. Mars and the Moon and Venus... the only planets that we've landed on—are utterly lifeless.... in our solar system we may discover that there is life only on this world. This says that life is not guaranteed, that life requires something special, something improbable.

The Varieties of Scientific Experience: A Personal View of the Search for God (2006)