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Intentionality, first systematically analyzed by Franz Brentano, is the property of mental states of being 'about' or 'directed toward' something — an object, state of affairs, or content. Beliefs are beliefs about something; desires are desires for something; fears are fears of something. Crucially, the object of an intentional state need not exist: one can fear ghosts, believe in Santa Claus, or desire perpetual motion. Brentano held intentionality to be the mark of the mental — what distinguishes mental from physical phenomena. The challenge for physicalism is to naturalize intentionality: to explain in purely physical terms how a brain state comes to be about anything at all.
Study Brentano's original distinction, then examine naturalistic theories of content: causal/informational theories (Dretske, Fodor) hold that a state represents X if it is causally connected to X in the right way; teleosemantic theories (Millikan) ground content in biological function.
Intentionality is one of the oldest puzzles in philosophy of mind, and it begins with a deceptively simple observation: mental states are always *about* something. When you believe, you believe *that* something is the case. When you desire, you desire *something*. When you perceive, you perceive *something*. This directed quality — the way mental states point beyond themselves toward an object or content — is what Brentano called intentionality, and he argued it was the defining mark of the mental: what separates minds from rocks.
The puzzle deepens immediately when you ask what the object of an intentional state actually is. If you believe the Loch Ness Monster exists, your belief is about Nessie — but Nessie (probably) doesn't exist. How can a state be *about* something that isn't there? Physical relations require relata that exist: a ball can't be to the left of a table that doesn't exist. But mental states routinely point at fictions, impossibilities, and non-existents. This asymmetry suggests intentionality is not an ordinary physical relation — which is exactly why it troubles physicalists.
It's crucial to separate intentionality (a structural feature of all mental states) from ordinary *intention* (a deliberate plan to act). Perceiving a red wall, remembering a childhood smell, and fearing the dark are all intentional states in Brentano's sense — they all have content, they are all *about* something — yet none of them are "intentions" in the everyday sense. The philosophical concept is far broader.
For physicalists, the project is naturalization: explaining how brain states acquire semantic content using only the resources of physics, chemistry, and biology. Causal/informational theories (Dretske, Fodor) try to ground content in the right kinds of causal connections between brain states and the world — a state "represents" X if it reliably covaries with X. Teleosemantic theories (Millikan) ground content in biological function: a state means X if it was selected for because it tracks X. Both approaches face hard cases involving error and misrepresentation — if my state was caused by a fox, does it mean *fox*, or does it mean whatever normally causes it?
Understanding intentionality matters because it underpins debates about representation, consciousness, artificial intelligence, and the Chinese Room argument. If a system can process symbols without any genuine aboutness — without its states really meaning anything — then symbol manipulation alone may not be sufficient for mind. Whether a silicon computer could ever have genuinely intentional states, or only simulate them, depends on what intentionality ultimately is and whether it can be naturalized.
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