One of the most deeply engrained habits of the modern world is the habit of thinking in terms of a contrast, and indeed of an opposition, between something we call Mind and something we call Matter. This habit is obviously not confined to philosophy. It is built into the structure of our languages and of our ways of behaviour. It conditions our religious and moral attitudes, as well as our reflective thought in science and philosophy. It is not surprising, therefore, that much of modern European Philosophy has consisted in an effort to overcome the dualism to which this habit gives rise when we try to clarify and systematize our knowledge of Reality. This essay on the subject is an essay in scepticism.