This book offers a comprehensive and broadly rationalist theory of the mind which continually tests itself against experimental results and clinical data. Taking issue with Empiricists who believe that all knowledge arises from experience and that perception is a non-cognitive state, Norton Nelkin argues that perception is cognitive, constructive and proposition-like. Further, as against Externalists who believe that our thoughts have meaning only insofar as they advert to the world outside our minds, he argues that meaning is determined 'in the head'. Finally, he offers an account of how we acquire some of our most basic concepts, including the concept of the self and that of other minds.