One has to be careful with “ism” words. They often have both loose and strict meanings. And sometimes multiple meanings of each type. ‘Behaviorism’ is no exception. Loosely speaking, behaviorism is an attitude – a way of conceiving of empirical constraints on psychological state attribution. Strictly speaking, behaviorism is a doctrine – a way of doing psychological or behavioral science itself.Wilfred Sellars (1912–89), the distinguished philosopher, noted that a person may qualify as a behaviorist, loosely or attitudinally speaking, if they insist on confirming “hypotheses about psychological events in terms of behavioral criteria” (1963, p. 22). A behaviorist, so understood, is someone who demands behavioral evidence for any psychological hypothesis. For such a person, there is no knowable difference between two states of mind (beliefs, desires, etc.) unless there is a demonstrable difference in the behavior associated with each state. Consider the current belief of a person that it is raining. If there is no difference in his or her behavior between believing that it is raining and believing that it is not raining, there is no grounds for attributing the one belief rather than the other. The attribution is empirically empty or unconstrained.Arguably, there is nothing truly exciting about behaviorism loosely understood. It enthrones behavioral evidence, an arguably inescapable premise not just in psychological science but in ordinary discourse about mind and behavior. Just how behavioral evidence should be ‘enthroned’ (especially in science) may be debated. But enthronement itself is not in question.Not so behaviorism the doctrine. It has been widely and vigorously debated. This entry is about the doctrine, not the attitude. Behaviorism, the doctrine, has caused considerable excitation among both advocates and critics. In a manner of speaking, it is a doctrine, or family of doctrines, about how to enthrone behavior not just in the science of psychology but in the metaphysics of human and animal behavior.Behaviorism, the doctrine, is committed in its fullest and most complete sense to the truth of the following three sets of claims.Psychology is the science of behavior. Psychology is not the science of the inner mind – as something other or different from behavior.Behavior can be described and explained without making ultimate reference to mental events or to internal psychological processes. The sources of behavior are external (in the environment), not internal (in the mind, in the head).In the course of theory development in psychology, if, somehow, mental terms or concepts are deployed in describing or explaining behavior, then either (a) these terms or concepts should be eliminated and replaced by behavioral terms or (b) they can and should be translated or paraphrased into behavioral concepts.The three sets of claims are logically distinct. Moreover, taken independently, each helps to form a type of behaviorism. “Methodological” behaviorism is committed to the truth of (1). “Psychological” behaviorism is committed to the truth of (2). “Analytical” behaviorism (also known as “philosophical” or “logical” behaviorism) is committed to the truth of the sub-statement in (3) that mental terms or concepts can and should be translated into behavioral concepts.Other nomenclature is sometimes used to classify behaviorisms. Georges Rey (1997, p. 96), for example, classifies behaviorisms as methodological, analytical, and radical, where “radical” is Rey’s term for what is here classified as psychological behaviorism. The term “radical” is instead reserved for the psychological behaviorism of B. F. Skinner. Skinner employs the expression “radical behaviorism” to describe his brand of behaviorism or his philosophy of behaviorism (see Skinner 1974, p. 18). In the classification scheme used in this entry, radical behaviorism is a sub-type of psychological behaviorism, primarily, although it combines all three types of behaviorism (methodological, analytical, and psychological).