This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book focuses on sexual difference, the difference between men and women and argues that this difference has not been sufficiently recognized or acknowledged within Western thought. It considers the centrality of binary logic for Western philosophical thinking exemplified by Parmenides, and shows how Luce Irigaray appeals to Empe-docles to destabilize the hegemony of the binary system and to introduce the idea of a fluid logic. The book also considers the same binary system with its implicitly male subject in its psychoanalytic manifestation. It revisits the rigid binary systems of philosophy and psychoanalysis and also shows how these fields of thought could be enriched by intellectual and subjective formation that is forever fluid.