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of anything else, and is the recipient of qualities, as for example a man, a mountain, a chair. Or it is a quantity, like four, or cubit; or a quality, like good, black, straight; or a relation like long, double, master, slave; and so on throughout the ten categories. This classification applies to words and thoughts as well as to things. As an analysis of the first two it led him to more important investigations of speech and thinking and arguing, and resulted in his system of logic, which is the most momentous discovery of a single mind recorded in history. As applied to things it was followed by a more fundamental analysis of all real objects in our world into the two elements of matter and form. He argued as follows: nothing in the material world is permanent as an individual thing. It changes its state from moment to moment and finally ceases to be the thing it was. An acorn passes a number of stages before it is ripe, and when it is placed in the ground it again changes its form continually and then comes out as an oak. In artificial products man in a measure imitates nature. He takes a block of marble and makes a statue out of it. He forms a log into a bed. So an ignorant man becomes civilized and learned. All these examples illustrate change. What then is change? Is there any similarity in all the cases cited? Can we express the process of change in a formula which will apply to all instances of change? If so, we shall have gained an insight into a process of nature which is all-embracing and universal in our experience. Yes, we can, says Aristotle. Change is a play of two elements in the changing thing. When a thing affected with one quality changes into a thing with the opposite quality, there must be the thing itself without either of the opposite qualities, which is changing. Thus when a white fence becomes black, the fence itself or that which undergoes the change is something neither white nor black. It is the uncolored matter which first had the form of white and now lost that and took on the form of black. This is typical of all change. There is in all change ultimately an unchanging substratum always the same, which takes on one quality after another, or as Aristotle would say, one form after another. This substratum is matter, which in its purity is not affected with any quality or form, of which it is the seat and residence. The forms on the other hand come and go. Form does not change any more than matter. The changing thing is the composite of matter and form, and change means separa-