Brentano’s Thesis not entirely clear, but its importance should be obvious. Suppose that every psychological fact somehow involves something with intentional inexistence (however precisely this is to be construed). Suppose that no physical fact involves a thing with intentional inexistence. Then it would follow that no psychological fact can be identified with a physical fact; one version of the psycho-physical identity theory would be false; physicalism of some popular varieties would be untenable.