31 This fact gives rise to one of the main problems of intentionality. For an excellent presentation of this problem, see Dummett, Michael, Origins of Analytical Philosophy (London: Duckworth, 1993), pp. 35–6Google Scholar. See also Caston, ‘Aristotle’. Brentano came to appreciate the importance of this point when he wrote the appendix to his Psychology from an Empirical Standpoint. There he says that ‘If someone thinks of something, the one who is thinking must certainly exist, but the object of his thinking need not exist at all.’ He goes on to observe that ‘we might doubt whether we are dealing with something relational here, and not, rather, with something somewhat similar to something relational in a certain respect, which might therefore better be called “quasi-relational”‘ (p. 272). Sometimes it is supposed (see Dennett, Content, and Tye, Ten Problems) that Brentano was concerned with the question of non-existence even before he wrote the Appendix to his Psychology. It is true that in a famous passage, Brentano says that the object of thought ‘should not be understood as a reality’ (p. 88); but by this he is just reminding his readers that he is talking about ‘phenomena’ or ‘appearances’, not about the ‘underlying reality’. In this sense, the physical phenomena with which he contrasts mental phenomena ‘should not be considered a reality’ either. Compare, for example, the following passage: ‘the phenomena of light, sound, heat, spatial location and locomotion which [ the natural scientist ] studies are not things which really and truly exist. They are signs of something real, which through its causal activity, produces presentations of them. They are not however, an adequate representation of this reality… We have no experience of that which truly exists, in and of itself, and that which we do experience is not true.’ (Brentano, Psychology, p. 19).