The cogito ergo sum of Descartes is one of the best-known (and simplest) of all philosophical formulations, but ever since it was first propounded it has defied any formal accounting of its validity. How is it that so simple and important an argument has caused such difficulty and such philosophical controversy? In this pioneering work, Jerrold Katz argues that the problem with the cogito lies where it is least suspected--in a deficiency in...