In recent decades gender identity theory has surged from queer obscurity to mainstream prominence. The basic idea is that "gender" is a personal and performative identity category, and one's "sex" is defined by one's gender, not the other way around. Further, this is not just a theory, it is a political and moral imperative. In the cause of justice for the marginal and a diverse and inclusive utopia, we must now discard oppressive and supposedly "objective" commonsense conventions premised on the male/female sex binary. Thus, gender theory, in the form it has now come to be understood, purports to justify the tearing up of sex-defined gender norms and outlaws sex-based rights for women and girls. But where does this theory come from? Is it persuasive? Gender theory is the intellectual progeny of Kant's Enlightenment. While modern and postmodern philosophy after Kant really is persuasive if one accepts Kant's purity agenda, by now it is clear that this pathway leads to madness. Purifying science from faith, and purifying reason from metaphysics, leads to the inability to tell the difference between a man and a woman. This book seeks to show the reader why Kant was wrong and why gender theory is wrong.