Brentano considers a variety of ways in which one might try to distinguish the mental from the physical. The one of main interest to us is the one he presents in section 5 (pp. 481f) and again in the concluding section 9.

The terms he uses to present this could be confusing. The most important is "intentional." His use of the term (which, as he notes, has roots in medieval Latin) does not mean the aim or purpose of an action (though it is etymologically related to that concept) but rather the directedness of thought, what Brentano speaks of as "direction toward an object." This use of "object" and the use of "objectivity" in the phrase "immanent objectivity" could also be confusing. "Object" here refers to an object of thought; that's why "direction toward an object" is roughly synonymous with "reference to a content"; and "objectivity" refers having an object in that sense. Finally "inexistence" in the phrase "intentional inexistence" does not mean "non-existence" but rather existence in something (in the way for which the terms "inherence" or "immanence" are also used). So "intentional inexistence" and "inherent objectivity" are meant to be synonymous, with "inexistence" amounting to "inherence" and "objectivity" amounting to "intentionality."

Chisholm offers an extended explication of the idea of intentionality in his section 2. Think through each of the three "marks" he offers there. These are intended to be only sufficient conditions for intentionality; that is, it is enough to have one of these marks, and there are many examples of sentences that will have one but not others. (For example, the phrase "knows that" typically introduces intentionality because of the third mark; but to say that someone knows that a propositional clause is true implies that the propositional clause is true, so it doesn't exhibit the second mark.)

Chisholm uses this characterization of intentional language to state what he takes to be the content of Brentano's characterization of the mental (pp. 485f). The rest of Chisholm's paper is devoted to three attempts, all roughly behaviorist, to show that this claim is wrong by showing that it is possible to describe mental phenomena without using intentional language. Discussing these attempts and Chisholm's reasons for thinking they fail is likely to occupy the bulk of the class on Tuesday.