This chapter discusses James’s empiricism, the theoretical and practical viewpoints from which he developed his Principles of Psychology, and the methods he used to gather evidence and offer interpretations. Along the way it treats his tendency to focus on the close observation and description of mental states, his cautious approach to theorizing, and his views about introspection, experimentation, and the comparison of different species and different categories of human subjects. Also discussed are the caveats he articulated regarding the use of language, possible perceptual biases, and the need to avoid “the psychologist’s fallacy” of mistaking the investigator’s perspective for the phenomena under study.