‘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.