This transparent clearness in the observation of our thought-processes is quite independent of our knowledge of the physiological basis of thought. I am speaking here of thought in the sense in which it is the object of our observation of our own mental activity. For this purpose it is quite irrelevant how one material process in my brain causes or influences another, whilst I am carrying on a process of thought. What I observe, in studying a thought-process, is, not what process in my brain connects the concept of thunder with that of lightning, but what is my reason for bringing these two concepts into a definite relation. Introspection shows that, in linking thought with thought, I am guided by their content, not by the material processes in the brain. This remark would be quite superfluous in a less materialistic age than ours. To-day, however, when there are people who believe that, when we know what matter is, we shall know also how it thinks, it is necessary to affirm the possibility of speaking of thought without trespassing on the domain of brain physiology. Many people to-day find it difficult to grasp the concept of thought in its purity. Anyone who challenges the account of thought which I have given here, by quoting Cabanis' statement that "the brain secretes thoughts as the liver does gall or the spittle-glands spittle, etc." simply does not know of what I am talking. He attempts to discover thought by the same method of mere observation which we apply to the other objects that make up the world. But he cannot find it in this way, because, as I have shown, it eludes just this ordinary observation. Whoever cannot transcend Materialism lacks the ability to throw himself into the exceptional attitude I have described, in which he becomes conscious of what in all other mental activity remains unconscious. It is as useless to discuss thought with one who is not willing to adopt this attitude, as it would be to discuss colour with a blind man. Let him not imagine, however, that we regard physiological processes as thought. He fails to explain thought, because he is not even aware that it is there. For every one, however, who has the ability to observe thought, and with good will every normal man has this ability, this observation is the most important he can make. For he observes something which he himself produces. He is not confronted by what is to begin with a strange object, but by his own activity. He knows how that which he observes has come to be. He perceives clearly its connections and relations. He gains a firm point from which he can, with well-founded hopes, seek an explanation of the other phenomena of the world.