Early forms of psychology assumed that mental life was the appropriate subject matter for psychology, and introspection was an appropriate method to engage that subject matter. In 1913, John B. Watson proposed an alternative: classical S-R behaviorism. According to Watson, behavior was a subject matter in its own right, to be studied by the observational methods common to all sciences. Unfortunately, by around 1930, Watson's behaviorism had proved inadequate. Many researchers and theorists then adopted a view in which various organismic entities were inferred to mediate the relation between S and R: mediational S-O-R neobehaviorism. This general view has remained influential, although the details of the various versions have differed over the years. The behavior analysis of B. F. Skinner took an entirely different approach. Particularly important was the study of verbal behavior. Although behaviorism is often conventionally defined as an approach that seeks to explain behavior without directly appealing to mental or cognitive processes, this definition needs considerable clarification, especially as it pertains to Skinner's behavior analysis and his view of behaviorism as a philosophy of science.