Since the days of Aristotle the general terms used in reasoning have been called categories. Any class-term or genus may be called a category. There are certain _summa genera_, the most extensive classes or classes with the lowest connotation, that have been traditionally known as categories, because everything that can be affirmed in a judgment must come under some one or other of them. Aristotle names ten,――substance, quality, quantity, etc. But these Aristotelian categories are classes of analytical relations, such as formal logic treats. They are the classes of the attributes and relations into which objects may be analyzed. These evidently are not what Kant is seeking. He is in search of synthetic categories. He is looking for the synthetic forms of the understanding itself, which transform perceptions into objects of knowledge. He is not looking merely for abstract conceptions. For ideas become nature objects only when they are thought as things with qualities universal to every human mind. The understanding creates out of the perceptions the objects of thought which form the nature-world; and the categories of the understanding are the constitutive principles of such objects. The categories are the relating forms of synthesis through which objects arise. The most difficult part of the _Critique_ is called the “Deduction of the Categories,” in which Kant attempts to derive the synthetic forms of the understanding from the various kinds of judgment. Kant’s list is curious but unimportant, and only two of these categories are useful,――substance and cause. He divides the categories into four general kinds and enumerates three categories of each of these kinds, as follows:―― Categories of Quantity,――Unity, Plurality, Totality. Categories of Quality,――Reality, Negation, Limitation. Categories of Relation,――Substance, Cause, Reciprocity. Categories of Modality,――Possibility, Existence, Necessity. These categories occupy the same position in the understanding that space and time do in the perception,――they are the _a priori_ principles. In respect to them the perceptions are the _a posteriori_ material. The categories are pure, innate, and transcendental. They are the inner nature of the understanding. Thus the objects of the understanding contain both _a priori_ and _a posteriori_ factors, and are syntheses of manifolds. Perception synthesizes sensations, while the understanding synthesizes perceptions, and states the synthesis in the form of a judgment.