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How did Franz Brentano distinguish mental events from nonmental things?

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Bronze bust of Epicurus with a full beard and draped clothing, displayed on a round inscribed base at the National Archaeological Museum of Naples. — Materialism Epicurus, bronze bust from a Greek original, c. 280–270 bce; in the Museo Archeologico Nazionale, Naples.

The German philosopher Franz Brentano distinguished mental events from nonmental things by identifying intentionality as the defining mark of the mental.12

Brentano argued that all mental states—such as beliefs, thoughts, feelings, and perceptions—are inherently "about" something. This property, which he described as a "direction of the mind to an object," means that mental acts possess "intentional content" or "immanent objectivity," where an object is said to "inexist" within the mental act itself. In contrast, nonmental things like stones or stars do not represent anything and lack this intrinsic reference to an object.13

Brentano’s thesis posits that this intentionality is a fundamental characteristic of all mental phenomena. He maintained that because mental states can point toward objects—even those that do not exist in reality, such as Santa Claus—they possess a unique nature that cannot be reduced to purely physical terms. Consequently, he viewed intentionality as the essential criterion that separates the mental realm from the physical world.12