Both Plato and Aristotle, although they disagree about whether poetry is ultimately good, both judge all forms of artistic endeavor by their effects on the moral constitution of the polis.

The problem with the fall of the virtuous man is that such a protagonist would have done nothing to deserve his bad fate. Essentially, such a portrayal would have no positive moral impact on the audience, but rather the opposite as it would show fate and the gods to act capriciously.

Comedy deals with people who are essentially ignoble, and by satirizing their faults, educates the audience. Thus the fall of ignoble characters is not a subject for tragedy because we feel they deserve their fate and do not feel fear or pity, but laugh at them or celebrate their downfall.

The appropriate subject of tragedy is the downfall of a great, noble, but flawed hero such as Oedipus. According to Aristotle, we sympathize with such a hero, and thus feel fear and pity at his downfall, but we also see how his inherent character flaws (usually arrogance) lead to this downfall.