Thinking is the activity of our mind which discovers the order, arrangement, and system in the reality that the senses reveal. Without thought, our felt experience would be a chaos and not a world. The philosopher Kant expressed this by saying that the understanding gives unity to the manifold of sense. The understanding, he said, makes nature. It does this by giving form to the matter which comes to it by the senses. The mind is not a _tabula rasa_ upon which the external world makes and leaves impressions, it is a relating activity which arranges the matter it receives in forms. First of all there are space and time, which are forms in which we receive all perceptual experience, and then there are categories that are conceptual frames or moulds by which we think of everything we experience as having definite relations and belonging to a real order of existence. Substance, causality, quality, and quantity are categories; they are universal forms in which the mind arranges sense experience, and which constitute the laws of nature, the order of the world. Space and time, and the categories of the understanding Kant declared to be transcendental--that is to say, they are the elements necessary to experience which are not {27} themselves derived from experience, as, for example, that every event has a cause. There are, he declared, synthetic _a priori_ judgments--that is, judgments about experience which are not themselves derived from experience, but, on the contrary, the conditions that make experience possible. It is from this doctrine of Kant that the whole of modern idealism takes its rise. Kant, indeed, held that there are things-in-themselves, and to this extent he was not himself an idealist, but he also held that things-in-themselves are unknowable, and this is essentially the idealist position. Clearly, if we hold the view that things-in-themselves are unknowable, truth cannot be a correspondence between our ideas and these things-in-themselves. Truth must be some quality of the ideas themselves, and this can only be their logical consistency. Consistency, because the ideas must be in agreement with one another; and logical, because this consistency belongs to the thinking, and logic is the science of thinking. Truth, in effect, is the ideal of logical consistency. We experience in thinking an activity striving to attain the knowledge of reality, and the belief, the feeling of satisfaction that we experience when our thinking seems to attain the knowledge of reality, is the harmony, the absence of contradiction, the coherence, of our ideas themselves. This is the coherence theory. Let us see what it implies as to the ultimate nature of truth and reality.