David Hartley's importance for philosophy lies in his Observations on Man, his Frame, his Duty, and his Expectations (1749), which includes the first fully worked-out presentation of an associationist theory of mind. It makes a suggestion as to the physical basis of thought, and it defends a determinist conception of action. In addition, it presents a strikingly optimistic vision of the future of mankind and argues that the entire human race will ultimately recover the happiness lost at the Fall. While it is no longer widely read, Observations on Man, his Frame, his Duty, and his Expectations had a significant influence upon the poet and philosopher Samuel Taylor Coleridge, as well as upon the political radicalism of the early nineteenth century and the development of the discipline of experimental psychology. Hartley was one of the first writers in English to use the word "psychology" in its modern sense, and...