Whether mental states are limited to conscious experience or encompass unconscious states was arguably the single most important and divisive question to confront psychology at its inception as a science. Both psychology and philosophy of mind changed radically as a result of the ensuing debate, from understanding mind as synonymous with consciousness in the nineteenth century to the present understanding of mind as unconscious representational states realized in brain tissue. The puzzle is: By what arguments, were these transformations justified? Freud's philosophical argument for the existence of unconscious mental states, excavated and reconstructed in this book, is proposed as a "missing link" that provides a possible rationale for the transformations of psychology and philosophy of mind.