“Know thyself.” Once carved into the stone at Delphi and popularized by Socrates as the guiding maxim of philosophy, this command captures the enduring human struggle to examine and justify our own beliefs. This book asks how such a capacity for self-knowledge could have arisen—our ability to reflect on and regulate what we believe in light of epistemic norms like justification and truth. To answer that question, it weaves together insights from comparative and developmental psychology, co...