What Aristotle meant by “pleasure proper to tragedy” is the appropriate set of pleasurable emotions elicited by a tragic drama.
In his Poetics, Aristotle argues that tragedy should elicit specific emotions, namely pity and fear. On the face of it, it seems rather strange to describe pity and fear as being in any way pleasurable. Surely, we might think, they’re anything but.
However, Aristotle insists that they most certainly are, because in experiencing them we cleanse our souls by way of purgation. In other words, as we watch a tragic play, the pity and fear in our souls are expelled in a process that Aristotle calls catharsis.
As a trained physician, Aristotle understood the importance of getting rid of bodily impurities for the overall health of the body. His theory of catharsis applies in much the same way to the soul. In eliciting fear and pity, and so purging them from our souls, tragic drama administers a kind of medicine to the soul that leaves us with a feeling of pleasure.