Chapter xiii insists that the vital principle of plot is causal consistency. This rules out mere reversal. A turning-point (περιπέτεια) is, indeed, characteristic of drama. There is usually and typically a crisis, in which the hero’s fortunes turn from good to bad; but this reversal will not suffice by itself. The mere turn of fortune does not achieve the catharsis of pity and fear. There remains, then, the [hero] between [the typically virtuous man and the typically depraved], a man neither exceptional in virtue and righteousness nor falling into adversity by vice and depravity, but by some error, a man among those who live in renown and prosperity, such as Œdipus or Thyestes or other illustrious men of such families. The perfect [tragic] plot, therefore, must be single, not, as some say, double;[37] the change of fortune not from adversity to prosperity, but on the contrary from prosperity to adversity; not through depravity, but through great error on the part of a man either such as we have described or rather better than worse.[38] Why this insistence on character in the midst of the discussion of plot? Why the iteration of “not through depravity, but through error”? Because, as Aristotle shows below,[39] plot implies consistency of characterization, but more fundamentally because consistency of plot has for its very beginning and mainspring the realization of a central figure like ourselves progressively winning our sympathy. The essence of plot is motivation. What moves us is never mere luck, never mere surprise, but the causation that springs from human will. Consistency of plot means clear causation; and causation in drama is the working of will. So the first consideration is the title rôle, the main “part.” He or she should be illustrious because the action is thereby conspicuous and partly known in advance; but his course of action must be moved by springs that we feel in ourselves. Macbeth is a warrior of an elder day and a king; but we, though neither warriors nor kings, feel the perversion of his manhood as like enough to our own to purify us through pity and fear. Fear and pity may, indeed, be aroused by mere spectacle, but they may also be aroused from the very plan of the actions,[40] and the latter is superior and shows a better dramatist. This is the second consideration of consistency. First, the best tragedy springs from a great personal will gone wrong; secondly, it springs from a compelling progress of actions, from the plot itself. It depends not on the shock of this violent deed or that, but on the causal movement of the whole.