This work is an analysis of Moses Mendelssohn's contributions to aesthetic rationalism, a tradition that arose in 18-century Germany. Rationalists held that aesthetic experience is primarily explained by the perfection of the object being considered, where perfection is a fundamental, rational (law-governed) property. As this work shows, Mendelssohn was among the first to acknowledge and effectively address several significant objections to the rationalist theory: its seeming inability to account for pleasure generally, tragedy and tragic pleasure more specifically, and the sublime; and its apparent blindness to the claims of genius and Rousseau's ethical critique of the arts. Many commentators have claimed that Mendelssohn saw these issues as reasons to move away from aesthetic rationalism, but Mendelssohn in fact attempted to address each of them from within the rationalist framework. Mendelssohn’s resulting elaboration and defense of the rationalist tradition illustrates its resilience and lasting relevance.