Intentionality, originally a concept from scholastic philosophy, was reintroduced in contemporary philosophy by the philosopher and psychologist Franz Brentano in his work Psychologie vom Empirischen Standpunkte. While often simplistically summarised as "aboutness" or the relationship between mental acts and the external world, Brentano defined it as the main characteristic of "psychical phenomena" (psychische Phänomene), by which they could be distinguished from "physical phenomena" (physische Phänomene). Every psychical or mental phenomenon, every psychological act has a content, is directed at an object (the intentional object). Every belief, desire etc. has an object that they are about: the believed, the wanted. Brentano used the expression "intentional inexistence" to indicate the status of the objects of thought in the mind. The property of being intentional, of having an intentional object, was the key feature to distinguish psychical phenomena and physical phenomena, because physical phenomena lack intentionality altogether.
Through the works of Husserl, who took it over from Brentano, the concept of intentionality received more widespread attention in current philosophy, both continental and analytic.
In the field of social cognition, intentionality also has a different meaning. There actions are called intentional when they are purposeful, meant or done intentionally. People typically can read the intentions of behavior of others effortlessly.
N.B. intentionality (-tion-) is not to be confused with intensionality (-sion-), a concept from semantics.
