He was thinking about it as part of a social context, not as an art form in isolation: in many ways, I tempted to dismiss his conclusions because they tend to limit tragedy to a social action, a branch of not just literature but further education (or social indoctrination).
Bernays spots that the apparently definitive explanation of tragedy given by Aristotle has been complicated by his interpreters. Even a small word, like toiotoon (translated as 'such') can cause Lessing to get angry at previous interpretations. So, when he gets to the magic word - catharsis - he goes back The Politics to get some clarification.
And so, Politics VIII, 7.
We accept the division of melodies... into ethical melodies, melodies of action, and passionate and inspiring melodies, each having, as they say, a mode corresponding to it. But we maintain further that music should be studied, not ... with a view to education, to catharsis.
He then promises to explain the word later, but doesn't. He gets back onto the matter in hand.
In education, the ethical modes are to be preferred... for emotions such as fear and pity, or again enthusiasm, exist very strongly in some souls... Some persons fall into a religious frenzy, whom we see as a result of the sacred melodies... restored as though they had found catharsis or healing. Those who are influenced by fear and pity must have a like experience.
Bernays goes on to say that Aristotle did see theatre as for both the top dogs and the 'vulgar crowd' - unlike Plato who was against complex modern music - and that the element of catharsis should not overwhelm the need to entertain: the lower classes can get their jollies, too. Mind you, the point of the whole thing is still moral education, only sneaked in behind a big load of lower order carrying on...