It is a very wise rule in the conduct of the understanding, to acquire early a correct notion of your own peculiar constitution of mind, and to become well acquainted, as a physician would say, with your idiosyncrasy. Are you an acute man, and see sharply for small distances? or are you a comprehensive man, and able to take in, wide and extensive views into your mind? Does your mind turn its ideas into wit? or are you apt to take a common-sense view of the objects presented to you? Have you an exuberant imagination, or a correct judgment? Are you quick, or slow? accurate, or hasty? a great reader, or a great thinker? It is a prodigious point gained if any man can find out where his powers lie, and what are his deficiencies, — if he can contrive to ascertain what Nature intended him for.

Sydney Smith (1771-1845) English clergyman, essayist, wit

Lecture (1804-1806), Moral Philosophy, No. 9 “On the Conduct of the Understanding,” Royal Institution, London
    (Source)

Collected in Elementary Sketches of Moral Philosophy (1849).