Letter 88 — On Liberal and Vocational Studies (§25)

“But many things aid us and yet are not parts of ourselves. Nay, if they were, they would not aid us. Food is an aid to the body, but is not a part of it. We get some help from the service which mathematics renders; and mathematics is as indispensable to philosophy as the carpenter is to the mathematician. But carpentering is not a part of mathematics, nor is mathematics a part of philosophy.”