We have just two lousy, overused terms to work with, and both terms have too much play in them for precision work. This is an important book that I strongly urge all readers to study. Thus, agon is the term weuse to describe the activity of a runner at the Olympic Games, while paidia applies to a child throwing a ball. In the quote from Plato at the beginning of this essay, the Greek word that he used, translated into English as 'play', was 'paidia'. The catalyst that welds these two strangers together is interactivity.
All play requires interaction of some sort.
But as we grow, bouncing balls lose their fascination, and we seek out interactive partners capable of richer behavior. Some of us, mostly adolescent males, get sidetracked and fixate on computers as interactive partners. But most of us seek out other people as interactive partners, for the equality of interaction of human intercourse provides the most challenging and complete form of play. Interaction can only take place where there is a perceived disrepancy of volition. The various angles, energies, and spins with which the ball bounces back are perceived by the child as manifestations of its volition.