This study concerns the position of Saint Thomas Aquinas on human self knowledge ("the soul's knowledge of itself," in medieval idiom). Its main goal is to present a comprehensive account of Aquinas's philosophy of self knowledge, by clarifying his texts on this topic and explaining why he made the claims he did. A second objective is to situate Thomas's position on self awareness within general world, and specific thirteenth century, traditions concerning this theme. And a third is to apply Aquinas's approach and insights to selected and contemporary issues that involve self knowledge, such as the alleged paradoxes of self reflection and of "unconscious awareness." The primary approach is that of "critical narrative," which attempts to understand St. Thomas's texts by posing critical questions for them. While this questioning may expose certain texts as equivocal or unsupported, usually Thomas emerges as coherent, reasonable, and better understood. This work is serious scholarship that presumes reader interest in philosophical reflection and some background in medieval type thinking. On the other hand, the book is not narrowly specialized in Aquinas or a single methodology, but includes broad reference to worldwide traditions and attempts to integrate St. Thomas's approach into topics of contemporary interest.