In the Weight of Glory (1949) Lewis writes:

A man’s physical hunger does not prove that that man will get any bread: he may die of starvation on a raft in the Atlantic. But surely a man’s hunger does prove that he comes of a race which repairs its body by eating, and inhabits a world where eatable substances exist.   In other words, If I find in myself a desire which no experience in this world can satisfy, the most probable explanation is that I was made for another world.