Parts B and C have covered branches of philosophy that are concerned primarily with our knowledge of the external world. We now turn to what is often called ‘the inner world’. In using this term in the Introduction to Part B, I said that I would take it to mean that of which the human mind is aware when it scrutinises itself, together with the activities of self-scrutiny themselves. In other words, to talk of the inner world is (for example) to talk of acts of perception, reasoning, introspection, imagination, memory and will; it is also to talk of such states as intelligence, sensitivity, ambition and determination. All these are the concern of that branch of philosophy which is called ‘the philosophy of mind’. But in what precise way is it concerned with them?