   Chisholm (1957) argued that reports or descriptions of intentional or psychological phenomena cannot be reduced to (or eliminated in favor of) descriptions of behavior. The intensionality or non-extensionality of reports of intentionality was taken to show that descriptions and explanations of psychological phenomena cannot be described nor explained in terms of a vocabulary describing non-intentional phenomena. As Quine (1960, 220) put it, “there is no breaking out of the intentional vocabulary by explaining its members in other terms”. Chisholm (1957) took this conclusion to show the correctness of Brentano’s third thesis that intentionality is the mark of the mental.