It is misleading then to talk of thinking as of a 'mental activity'. We may say that thinking is essentially the activity of operating with signs. This activity is performed by the hand, when we think by writing; by the mouth and larynx, when we think by speaking; and if we think by imagining signs or pictures, I can give you no agent that thinks. If then you say that in such cases the mind thinks, I would only draw attention to the fact you are using a metaphor, that here the mind is an agent in a different sense from that in which the hand can be said to be the agent in writing.
If again we talk about the locality where thinking takes place we have a right to say that this locality is the paper on which we write or the mouth which speaks. And if we talk of the head or the brain as the locality of thought, this is using the 'locality of thinking' in a different sense.
-- Ludwig Wittgenstein, Preliminary studies for the "Philosophical investigations".