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 a psychological theorist who demands behavioral evidence for any psychological hypothesis. For such a person, there is no knowable difference between two states of mind unless there is a demonstrable difference in the behavior associated with each state.