The career of a young theoretical physicist consists of treating the harmonic oscillator in ever-increasing levels of abstraction.

Said during one of his lectures at Harvard University

As quoted in: Lectures of Sidney Coleman on quantum field theory (World Scientific Publishing, 2019), p. 21

Variation of this remark: I apologize, but theoretical physics is defined as a sequence of courses, each of which discusses the harmonic oscillator

Sidney Coleman - Lecture 2