"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: and such are the changes and chances of the world, and so difficult is it to ascertain our own understandings, or those of others, that most things are done by persons who could have done something else better.
If you choose to represent the various parts in life by holes upon a table, of different shapes, — some circular, some triangular, some square, some oblong, — and the persons acting these parts by bits of wood of similar shapes, we shall generally find that the triangular person has got into the square hole, the oblong into the triangular, and a square person has squeezed himself into the round hole. The officer and the office, the doer and the thing done, seldom fit so exactly, that we can say they were almost made for each other."
--Sydney Smith, Lecture IX: On the Conduct of the Understanding, (the origin of the phrase a square peg in a round hole)