Brentano’s lasting importance lies in his emphasis on the intentionality of conscious states and attitudes. He pointed out that thoughts, beliefs, hopes, desires, and the like—which Bertrand Russell (1872–1970) was to term “propositional attitudes”—are directed toward some object. For instance, if you are thinking about an apple then your intentional object is the apple you are thinking about; if you want a new car, it is the car you intend as an object of that desire.

Because physical states are not intentional in this way, intentionality is a basis on which what is mental can be distinguished from what is physical. Brentano identified three different kinds of intending: ideas, judgments, and the phenomena of love and hate. The last, also known as emotions and volitions, are directly related to morality.

Although an earlier version of Brentano’s doctrine—called “immanent intentionality”—suggested that the object intended was in some way literally in the mind, he later explained that although there is always a mental object for consciousness the object need not literally exist. The point is that one can think of a thing that does not exist. Objects of thought that do exist have “strict relations” with other objects that exist, whereas those that do not exist lack them.