Feynman has been called the "Great Explainer". He gained a reputation for taking great care when giving explanations to his students and for making it a moral duty to make the topic accessible. His guiding principle was that, if a topic could not be explained in a freshman lecture, it was not yet fully understood. Feynman gained great pleasure from coming up with such a "freshman-level" explanation, for example, of the connection between spin and statistics. What he said was that groups of particles with spin ½ "repel", whereas groups with integer spin "clump". This was a brilliantly simplified way of demonstrating how Fermi–Dirac statistics and Bose–Einstein statistics evolved as a consequence of studying how fermions and bosons behave under a rotation of 360°. This was also a question he pondered in his more advanced lectures, and to which he demonstrated the solution in the 1986 Dirac memorial lecture. In the same lecture, he further explained that <answer>antiparticles</answer> must exist, for if particles had only positive energies, they would not be restricted to a so-called "light cone".
In a lecture, Feynman said that what had to exist?
antiparticles