Centenary of William James's Principles of Psychology

William James's views on the science of psychology were as provocative as his observations of psychological and behavioral phenomena. His perspective on psychology as a natural science is reviewed briefly, the concept of "levels" is defined, and several emerging principles illustrating the importance of multilevel integrative research on mind-brain relationships are described. These emerging principles, which are not derived from, but would be at home in, James's Principles of Psychology, are the principle of multiple determinism, the corollary of proximity, the principle of nonadditive determinism, and the principle of reciprocal causation. These principles illustrate how the understanding of social psychological phenomena can inform and be informed by the study of both environmental (situational) and organismic (biological) factors.