Thinking for its own sake, as in music! When I have no special problem to occupy my mind, I love to reconstruct proofs of mathematical and physical theorems that have long been known to me. There is no goal in this, merely an opportunity to indulge in the pleasant occupation of thinking.

In: Helen Dukas and Banesh Hoffman, Albert Einstein: The Human Side: New Glimpses from His Archives, Letter from Spring 1918 (p. 17)