Brother of novelist Henry James and godson of eminent philosopher, essayist and poet Ralph Waldo Emerson, William James earned his own reputation for the monumental contributions he made in the field of Psychology. Here in his most comprehensive work on psychology, “Principles of Psychology”, we find a textbook which took James twelve years to complete, and which is still prevalent in the contemporary study of psychology. James’ writings were influenced by Charles Darwin’s ideas about adaptive evolutionary change, philosophical pragmatism, and various German psychologists who focused on the study of psychological processes. This work delves into James’ assumption that developmental processes involve an interaction of nature and nurture, a view that almost all contemporary developmental psychologists hold. His fundamental theories on brain processes and abstract thought, behavioral tendencies and states of consciousness, all of which he presented a decade before Sigmund Freud, have become integral to the framework of modern social psychology. Originally published in two volumes in 1890, the complete “Principles of Psychology” is collected together here in one volume. This edition includes a biographical afterword.