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The fundamental question of mental content is: what makes a thought about the Eiffel Tower rather than something else? Competing accounts exist—causal theories (thoughts are about what causes them), teleosemantic theories (thoughts are about what they evolved to represent), and externalist theories (thoughts get content from environment).
From your study of intentionality, you know that mental states have aboutness — they are directed toward objects, properties, and states of affairs. A belief is always a belief *that* something, a desire is always a desire *for* something. But intentionality raises an immediate puzzle: what in the physical world could possibly constitute this directedness? What makes a neural state be *about* Paris rather than about Buenos Aires, or about nothing at all?
The causal theory of content offers the most intuitive starting point: a mental state represents X if X is what reliably causes that state. If your "dog"-concept fires in the presence of dogs, then dogs are what it represents. This captures something right — we do expect content to track the environment — but it faces the problem of misrepresentation. If a thought can be *wrong*, then a mental state can occur without its normal cause. When you mistake a wolf for a dog, your dog-concept fires, but what causes it is a wolf. Does your thought therefore represent wolves? If content is fixed by what actually causes the state on a given occasion, there can be no such thing as error, which is clearly absurd.
Teleosemantic theories (Ruth Millikan, David Papineau) solve misrepresentation by shifting from actual causes to *proper functions* — what a state was selected to do over evolutionary or developmental history. A frog's tongue-snapping mechanism was selected because it caught flies; that is what it *represents*, even when it misfires and snaps at a pellet of black paper. Error is now possible: the mechanism misrepresents when it fires without its historically normal cause. The cost is that content becomes sensitive to evolutionary history in ways that seem remote from ordinary thought — and it is unclear whether the theory extends to learned concepts and complex propositional thought without becoming ad hoc.
Externalist theories (building on Putnam and Burge, which you may have encountered) argue that content is fixed not just by internal causal history but by the nature of the external environment. What your "water" thought is about is partly fixed by what water actually *is* — H₂O — regardless of whether you know chemistry. These three families of theory are not mutually exclusive; most contemporary philosophers draw on causal, functional, and environmental factors in combination. The theoretical terrain here connects directly to debates about narrow vs. wide content: whether mental content can be specified purely by internal states, or whether it essentially involves the world beyond the skin.
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