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 of ˇby 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 your attention to the fact that 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.