How did Franz Brentano argue that intentionality marks mental states?

In the philosophy of mind, the German philosopher Franz Brentano argued that intentionality serves as the defining "mark of the mental," distinguishing mental states from physical ones.12

Brentano proposed that all mental phenomena are characterized by a "direction of the mind to an object," a property he termed "intentional inexistence" or "immanent objectivity". This means that mental acts—such as perceiving, judging, loving, or hating—are inherently "about" something, even when the object of that thought does not exist in reality (e.g., thinking about Santa Claus).13

He contended that this directedness is a fundamental feature of consciousness, often summarized by the claim that "consciousness is always consciousness of something". Because he believed this intrinsic intentionality could not be explained or reduced to physical terms, he maintained that mental states possess a unique status that sets them apart from the purely physical world. This thesis remains a significant point of debate in contemporary discussions regarding materialism and the nature of the mind.12