What principle did Feynman use as the title of his PhD thesis?

What principle did Feynman use as the title of his PhD thesis? The principle of least action. His 1942 Princeton dissertation was titled “The Principle of Least Action in Quantum Mechanics.”

The principle of least action is a powerful way to describe physical motion. Instead of starting directly with forces, it identifies the path for which a quantity called the action is stationary—usually meaning that small changes to the path do not change the action to first order. In classical mechanics, this approach is closely associated with the work of Pierre-Louis Maupertuis, Leonhard Euler, Joseph-Louis Lagrange, and William Rowan Hamilton.

Feynman extended the idea into quantum mechanics. His formulation treated the transition between two states as receiving contributions from every possible path, with each path weighted according to its action. In the classical limit, paths near the stationary-action path reinforce one another, while other contributions tend to cancel.

This thesis became the foundation of the path-integral formulation of quantum mechanics. It also helped establish ideas that later influenced Feynman’s methods in quantum electrodynamics and his development of Feynman diagrams.