*Thought*.--Though thinking is impossible except by the use of imagery, to think is not merely to possess trains of imagery, or even to be aware of possessing them. Thinking means understanding the meaning of such mental imagery and arriving through the understanding at knowledge of the structure of the real world. How this process of interpreting mental imagery and reaching valid truth is achieved with greater and greater success until it culminates in the apprehension of the supreme principles of philosophy we have seen in dealing with the Aristotelian theory of knowledge. From the point of view of the "physicist" who is concerned with thinking simply as a type of natural process, the relation of "understanding" to the mental imagery just described is analogous to that of sensation to sensible qualities. The objects which thinking apprehends are the universal types of relation by which the world of things is pervaded. The process of thinking is one in which this system of universal relations is reproduced "by way of idea" in the mind of the thinker. The "understanding" thus stands to its objects as matter to form. The process of getting actually to understand the world is one in which our "thought" or "understanding" steadily receives completer determination and "form" from its contemplation of reality. In this sense, the process is one in which the understanding may be said to be passive in knowledge. It is passive because it is the subject which, at every fresh stage in the progress to knowledge, is being quite literally "informed" by the action of the real world through the sensation and imagery. Hence Aristotle says that, in order that the understanding may be correctly "informed" by its contact with its objects, it must, before the process begins, have no determinate character of its own. It must be simply a capacity for apprehending the types of interconnection. "What is called the intelligence--I mean that with which the soul thinks and understands--is not an actual thing until it thinks." (This is meant to exclude any doctrine which credits the "understanding" with either _furniture_ of its own such as "innate ideas," or a specific _structure_ of its own. If the results of our thinking arose partly from the structure of the world of objects and partly from inherent laws of the "structure of mind," our thought at its best would not reproduce the universal "forms" or "types" of interconnection as they really are, but would distort them, as the shapes of things are distorted when we see them through a lens of high refractive index.) Thus, though Aristotle differs from the modern empiricists in holding that "universals" realty exist "in" things, and are the links of connection between them, he agrees with the empiricist that knowledge is not the resultant of a combination of "facts" on the one side and "fundamental laws of the mind’s working" on the other. At