Hilbert’s 23rd Problem is the last in his famous list of problems and is of a different character than the others. The description is several pages, and basically says that the calculus of variations is a subject which needs development. We will look in retrospect at one of the critical events in the calculus of variations: The point at which the critical role of dimension was understood, and the role that the Palais-Smale condition(1963) played in this understanding. I apologize that in its present state, the talk consists mostly of my reminiscences and lacks references. I welcome suggestions from the audience.