Abstract
Herbert Spencer's Principles of Psychology (1855, first edition) was regarded by his contemporaries, including William James and John Dewey, as a major contribution to what was then a very new discipline. In this book he first expounded his ideas about both evolution of species and how behavior of the individual organism adapts through interaction with the environment. His formulation of the principle that behavior changes in adaptation to the environment is closely related to the version of the law of effect propounded some years later by Thorndike. He can thus be seen as the first proponent of selectionism, a key tenet of behavior analysis. He also explicitly attacked the then prevailing view of free will as being incompatible with the biologically grounded view of psychological processes that he was advocating, and thus put forward ideas that were precursors of B. F. Skinner's in this important area of debate.