This book considers one of the most fundamental, but only infrequently considered, issues in psychology--Are mental processes accessible by means of verbal reports and/or experimental assays? It is argues that this is the main characteristic distinguishing between behaviorism and mentalistic cognitivism. The answer posed by the author is that, with few exceptions and for the most fundamental reasons, mental processes are not accessible and that any psychology, such as contemporary cognitivism, based on a putative analysis of mind into its mental components must be fallacious. Classic and modern arguments against both mentalism and behaviorism are reviewed. In general, it is concluded that most antibehaviorist arguments are based on second order humanistic considerations rather than those underlying the usual scientific standards. Behaviorism represents the best that can be done in a situation of fundamental immeasurability and uncertainty. A modern version is offered in the final chapter of this book.