This book is not written for posterity. It is meant to constitute part of my contribution to a continuing debate at the intersection of the philosophy of mind, the philosophy of language, epistemology, and the philosophy of cognitive science. The debate is over how mental states are individuated. Many philosophers, call them externalists (or anti­ individualists), believe that the individuation of mental states requires appeal to an individual's social and/ or physical environment. I am not among them; I am an internalist ( or individualist). This book attempts to show that the leading proponents of externalism make a lot of mistakes in defending their view. This is either because the view is false, so any defense of it is bound to make some mistake or another, or it is because the people defending externalism have not been very careful, or both. I have included some introductory material so that those not already familiar with the issues involved might nevertheless gain entry into the debate. The intended audience, however, consists in those professional philosophers and students of philosophy who are already familiar with and interested in the question of how mental states are individuated. I do not spend much time trying to motivate an interest in this issue. Like all philosophy, it is interesting and worthwhile if one is blessed (or cursed) with the appropriate kind of intellectual curiosity. Those who do not meet this requirement proceed at their own risk.