How can you be a behaviorist and embrace private events?
Behaviorism was originally a movement against consciousness as the subject matter of psychology and introspection as the method of its investigation. Watson (1924, e.g., p. 14) claimed behavior as the subject matter of psychology and defined it by its form: behavior was muscle movements and glandular secretions. From his perspective, all activities of the organism could be reduced to these events (a kind of metaphysical behaviorism); and even if mental or other non-movement activities existed, they could not constitute the subject matter of a scientific psychology because public agreement as to their occurrence was impossible (a kind of methodological behaviorism). Thus for Watson, scientific legitimacy was an issue of public observability.
Skinner deviated considerably from these views. Skinner distinguished the subjective/objective dichotomy (which he thought to be of fundamental scientific importance) from the private/public dichotomy (which he thought was not fundamental). Skinner, in his 1945 paper on operationism, defined scientific observations as those under the control of a certain kind of contingency. Only when an observation was controlled by particular stimulus events (largely those of a nonverbal sort) and a general history of reinforcement for speaking under the control of those events, as opposed to control by audience factors, states of reinforcability, and so on, was the observation scientifically valid. As such, observations could be private and objective (scientifically legitimate) or public and subjective (scientifically illegitimate), depending upon the contingencies controlling the observations. This is the sense in which "radical behaviorism" is radical or "to the root": Even its core concepts and observations are defined in terms of contingencies, specifically, those bearing on the behavior of the scientist. Skinner rejected methodological behaviorism because he did not believe that public agreement provided assurance of proper contingency control.
As in the example above, it is easy to find instances where whole groups of observers are similarly influenced by motivational states and other subjective conditions. But in solving this problem by way of contingency analysis, Skinner opened up behaviorism to the very thing Watson was trying to eliminate: Introspective observations of private events. For example, Skinner said that radical behaviorism "does not insist upon truth by agreement and can therefore consider events taking place in the private world within the skin. It does not call these events unobservable" (Skinner, 1974, p. 16).
In a fundamental sense, radical behaviorism is not part of the tradition of "behaviorism" at all because all psychological activities that are contacted in a scientifically valid manner are subject to analysis. Thus contextual behavioral psychologists use the term "behavior" to mean something more like "psychological activity" than "behavior as distinct from thoughts and feelings." Skinnerians did not move rapidly to investigations of thinking and feeling for another reason, however. Skinner felt that an understanding of private events was not necessary for a scientific understanding of overt activity. He made that claim in essence because of his analysis of language in which the behavior of the listener is not verbal. From an RFT prespective this was an error and RFT corrects it. According to RFT, understanding thinking in the form of relational operants is essential to an understanding of overt human activity in most situations.