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Franz Brentano proposed that intentionality — the directedness or aboutness of mental states — is the defining mark of the mental. Every mental state is about something: beliefs are about states of affairs, desires are about outcomes, fears are about threats. Crucially, the objects of mental states need not exist: one can think about unicorns, fear nonexistent dangers, or desire impossible outcomes. Brentano called this 'intentional inexistence' — the object exists in the mental act without needing to exist in reality. This feature poses a deep puzzle for physicalism: physical states are not intrinsically about anything, so how can brain states possess this aboutness? Subsequent philosophy of mind has wrestled with whether intentionality can be naturalized or whether it remains irreducibly mental.
Begin with concrete examples: your belief that Paris is in France, your desire for coffee, your memory of yesterday. Notice how each has an object it is directed toward. Then consider cases where the object does not exist (Santa Claus, phlogiston). Read selections from Brentano's Psychology from an Empirical Standpoint, then trace how the concept is taken up by Husserl, Chisholm, and Searle.
You already know from your study of intentionality that mental states are directed — a belief is always a belief *about* something, a fear is always a fear *of* something. Brentano's contribution was to argue that this directedness — this aboutness — is the defining mark of the mental as such. If you want a criterion that separates mental from non-mental states, intentionality is it: rocks don't represent anything, but even the simplest mental state points beyond itself to something else.
The feature Brentano called intentional inexistence sharpens this claim in a philosophically disorienting way. Consider thinking about Sherlock Holmes, fearing a monster under the bed, or desiring the impossible. In each case, the object of your mental state does not exist — yet your mental state is still *about* that nonexistent thing. Physical processes don't behave this way. A rock's physical shape doesn't point to something that isn't there. This asymmetry between mental and physical is what makes intentionality philosophically puzzling: if the brain is a physical system, and physical systems aren't intrinsically about anything, how do brain states manage to be about something?
This is the naturalization problem. You've encountered the mind-body problem before — the question of how mental and physical relate. Intentionality adds a specific sharpness to that problem. It's not just that conscious experience seems non-physical; even the bare representational content of a thought — what it's *of* — seems to resist reduction to physical description. A physicist describing your brain can tell you which neurons are firing; that description, by itself, says nothing about what your thoughts are about. The physicist's description underdetermines the content.
Different philosophers respond to this differently. Functionalists try to ground content in causal-functional roles — your belief that there is water in the glass is that state because it is caused by water and causes water-seeking behavior. Externalists (which you'll study next in the course) argue that content is partly determined by the environment outside the head, not just internal states. Eliminativists bite the bullet and deny that folk-psychological intentional states are real. What unites these debates is Brentano's original provocation: there is something philosophically singular about the *aboutness* of mind, and any adequate theory of mind must account for it.
The key distinction to hold onto is between intentionality (directedness toward an object, including a nonexistent one) and mere causation (physical states that are caused by objects). A thermometer responds to temperature, but it doesn't represent temperature in the philosophically loaded sense — it doesn't misrepresent if miscalibrated in the way that a belief can be false. Representation involves a normative dimension, a standard of correctness, that seems hard to locate in purely physical causal relationships. Whether this difficulty is a deep metaphysical barrier or merely an engineering challenge is the live question that courses in naturalized epistemology and philosophy of language will continue to probe.
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