In the previous chapter, we argued that what makes a bit of discourse (overt or covert) count as an instance of thought is a matter of its setting – more specifically it is a question of the purposes which are in play and the capacities for understanding which are engaged. We intentionally left it vague how the relevant purposes and capacities relate to the discourse, but it would be entirely in keeping with the spirit of our earlier discussions to take the connections here to be causal ones. A bit of discourse (or symbol-shuffling, or piano-playing, or whatever) counts as exhibiting thought because it has the right kind of causes.

The notion of causality has been crucially woven into our theory of the mind ever since we endorsed a causal account of perception in Chapter VIII. Then in Chapter IX we arrived at the following view: intentional actions are things done for reasons in the light of which the agent's behaviour is comprehensible – in other words, an intentional action is something done because of appropriate beliefs and desires. It is entirely natural to take this, too, as a causal theory which identifies actions as events caused by beliefs and desires. This interpretation fits neatly with the position we reached in Chapter XII, where we argued that beliefs and desires are physical states identified precisely by their causal function in producing behaviour. Our argument surely implies that the relation between actions and the mental states which explain them must indeed be a causal one. Again, causality plays a crucial role in the functionalist theory of sensations sketched in Chapter XV.