Abstract
Throughout the philosophical tradition there usually have been those philosophers who have either denied the existence of mental entities outright, or else have claimed that they were, in some sense, reducible to physical entities. And, on this score, the twentieth century has been no exception. In the last twenty or so years, the various denials of the existence of mental entities have taken three distinct forms. First, there is the sort of behaviorism advocated by Quine and Ryle. Second, there is the materialism of J. J. C. Smart and D. M. Armstrong. And, third, there is the so-called Identity Theory as advocated by Donald Davidson, Thomas Nagel, and a host of others too numerous to mention.