‘It is of the very nature of consciousness to be intentional’ said Jean-Paul Sartre, ‘and a consciousness that ceases to be a consciousness of something would ipso facto cease to exist.’ Sartre here endorses the central doctrine of Husserl's phenomenology, itself inspired by a famous idea of Brentano's: that intentionality, the mind's ‘direction upon its objects’, is what is distinctive of mental phenomena. Brentano's originality does not lie in pointing out the existence of intentionality, or in inventing the terminology, which derives from scholastic discussions of concepts or intentiones. Rather, his originality consists in his claim that the concept of intentionality marks out the subject matter of psychology: the mental. His view was that intentionality ‘is characteristic exclusively of mental phenomena. No physical phenomenon manifests anything like it.’ This is Brentano's thesis that intentionality is the mark of the mental.
Despite the centrality of the concept of intentionality in contemporary philosophy of mind, and despite the customary homage paid to Brentano as the one who revived the terminology and placed the concept at the centre of philosophy, Brentano's thesis is widely rejected by contemporary philosophers of mind. What is more, its rejection is not something which is thought to require substantial philosophical argument. Rather, the falsity of the thesis is taken as a starting-point in many contemporary discussions of intentionality, something so obvious that it only needs to be stated to be recognized as true.