That a given substance which is a poison causes illness is a contingent matter. Poisons cause illness. There is a conceptual connection between being a poison and causing illness. Causal connections are contingent.
Mental states cause behaviour.
There is a conceptual connection between being in a mental state and behaviour. A functionalist theory holds that mental states are inner states which occupy or fill the roles specified by the clauses mentioned in 1. 3. A functionalist theory holds that quite different kinds of things could occupy or fill the roles specified in 1. Although functionalism is probably the most widely held theory of the mind amongst philosophers and cognitive scientists, there are many different kinds of functionalism, and much disagreement about which kind should be preferred. What everyone agrees is that mental states are to be defined in terms of their function rather than in terms of their substance or constitution.