I defend the thesis that self-examination and self-knowledge of our mental states is necessary for ethical development. Most moral philosophers take this thesis to be a truism and have, consequently, failed to investigate it. But it is pressing for them to do so. In the last few decades there have been significant contributions to this topic by psychotherapists, cognitive scientists and social psychologists, contributions that both challenge and shed new light on the thesis that self-knowledge contributes to ethical development. Investigating these contributions leads to a better understanding of the different ways in which our souls are disunified; it helps us recognize how to deal with such disunities; and it contributes to understanding important aspects of ethical development and moral education. In the dissertation I respond to a number of objections to this thesis, showing that there is an internal connection between self-knowledge and rational agency. By arguing that rationality is essential to a flourishing life I establish that self-knowledge is essential for flourishing. A central part of my project consists in arguing that the person aspiring to live a flourishing life should attempt to know her mental states first-personally, as opposed to third-personally. In arguing for the importance of first-personal self-knowledge I take myself to do justice to an insight that is central to most forms of psychotherapy but which has been neglected within moral philosophy (and within social psychology and cognitive science). The distinction between these two forms of self-knowledge has been central to epistemology but their ethical significance has been overlooked. Thus, in addressing this distinction I bring into contact the work that epistemologists have done on self-knowledge and first-person authority with the work that moral philosophers have done in moral education and ethical development. I argue that it is only when a person has first-personal self-knowledge that she can aspire to unify rationally the competing elements in her life into a coherent whole.