Maimonides

If a person studies too much and exhausts his reflective powers, he will be confused, and will not be able to apprehend even that which had been within the power of his apprehension. For the powers of the body are all alike in this respect.

New poems no longer come to me with their prodigies of metaphor and assonance. Prose endures. I feel the circles grow smaller, and old age is a ceremony of losses, which is, on the whole, preferable to dying at forty-seven or fifty-two.