Contrary to what some people think, a plot is not ipso facto a unity if it revolves about one man. Many things, indeed an endless number of things, happen to any one man some of which do not go together to form a unity, and similarly among the actions one man performs there are many that do not go together to produce a single unified action. Those poets seem all to have erred, therefore, who have composed a Heracleid, a Theseid, and other such poems, it being their idea evidently that since Heracles was one man, their plot was bound to be unified.