There are certain assumptions about the psychological domain that, if accepted, can greatly facilitate the development of a psychological system. These assumptions should be treated as axioms and regarded as true until disproved. Just as it is axiomatic within Euclidian geometry that a straight line is the shortest distance between 2 points, it can be useful for psychologists to treat as axiomatic the assumptions that people think and that what people think can determine their behavior. Without these assumptions, a great deal of justification is required that can impede the development of our field. Although axioms, by definition, cannot be proven, it would cause a great deal of mischief not to consider them to be true. It should be remembered, however, that the Euclidian axiom that a straight line is the shortest distance between 2 points was, in fact, eventually replaced by Einstein's notion of curved space. Similarly, when the cognitive axioms are refuted, we should not delay abandoning them. However, at this point in time, so much of our language and, we dare say, our thinking, is based on these axioms that we should simply assume they are true.