This study investigates the sources influencing Lessing's idea that tragedy improves the spectator morally by arousing his or her pity. Having shown that Lessing's comments on the tragedy's effect take into account three different problems (emotional effect, moral purpose and tragic pleasure), the study then identifies the central prerequisites for Lessing's model in both the humanist tradition (rhetoric, commentaries on Aristotle, poetics), and the epistemology of the Enlightenment (Leibniz, Wolff, Baumgarten, Hutcheson, Mendelssohn).