M ichelle M ontague have objects even though the objects which they can be said to have not in fact exist Diogenes could have looked for an honest man even if there hadn’t been any honest man.30 Chisholm elucidates this understanding of Brentano in what I will presently introduce as Chisholm’s ‘three marks of intentionality’, although he does so in terms of intentional sentences, thus conducting his discussion of intentionality in linguistic terms His central idea is that intentional sentences create intensional contexts An intensional context is a feature of sentences in which existential generalization and the principle of substituting co-referring terms fail Failures of existential generalization for simple declarative sentences and for sentences with propositional clauses constitute Chisholm’s first and second marks of intentionality, and the failure of the principle of substitution of co-referring terms constitutes his third mark of intentionality Existential generalization is a rule of inference in first order logic, for example, according to which atomic sentences such as ‘Fa’ (where ‘a’ stands for a singular term and ‘F’ stands for a predicate) entail that there exists an x such that x is F (‘∃xFx’) For example, “Bill is an honest man” entails that there exists an honest man Existential generalization fails in intensional contexts For example, a sentence such as “Diogenes looked for an honest man” is intensional, because neither it nor its contradictory “Diogenes didn’t look for an honest man” implies that there are or there are not honest men Chisholm extends this point to cover sentences with propositional clauses such as “James believes there are tigers in India” Neither this sentence reporting James’s belief nor a sentence reporting its contradictory implies that there are or there are not tigers in India According to the principle of substitution of co-referring terms, co-referring names and definite descriptions can be substituted in a sentence while preserving its truth-value For example, if ‘Muhammad Ali’ is substituted for ‘Cassius Clay’ in the sentence “Cassius Clay was a great boxer”, then the truth-value of the original sentence is preserved As already noted, this principle fails for certain intentional sentences We can’t validly infer that “Lois Lane believes that Clark Kent can fly” from the sentence “Lois Lane believes that Superman can fly” despite the fact that ‘Superman’ and ‘Clark Kent’ are co-referring terms After providing these three marks of intentionality, Chisholm claims that we may now re-express Brentano’s thesis – or something resembling that of Brentano – by reference to intentional sentences Let us say (1) that we not need to use intentional sentences when we describe nonpsychological phenomena; we can express all of our beliefs about what is merely “physical” in sentences which are not intentional But (2) when we wish to describe perceiving, assuming, believing, knowing, wanting, hoping and other such attitudes, then either (a) we must use sentences which are intentional or (b) we must use terms we not need to use when we describe nonpsychological phenomena.31 214