This chapter demonstrates how Ptolemy's distinctly mathematical ethics emerges from his response to a contemporary debate over the relationship between theoretical and practical philosophy. He first asserts that the two are independent, differentiated by the manner in which one attains virtues in each domain, whether by instruction or continuous activity. Thereafter, he diminishes the distinction by revealing how they relate. Theoretical philosophy, specifically mathematics, transforms the soul. The study of astronomical objects—the movements and configurations of heavenly bodies—reveals their constancy, good order, commensurability, and calm. Mathematicians, aided by habit, come to appreciate these qualities and transform their souls into a fine and well-ordered state. Organizing their actions in accordance with astronomical theories, they never forget their ultimate objective, the divine-like condition of the soul. The study of mathematics is crucial to obtaining this good life.