Full Idea
Exercise is the cause of fitness, but fitness is also the cause of exercise.
Gist of Idea
We exercise to be fit, but need fitness to exercise
Source
Aristotle (Metaphysics [c.324 BCE], 1013b10)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.115