Kant names the book the _Critique of Judgment_, or, more exactly, the _Critique of the Faculty of Judgment_. Judgment is distinguished from understanding, whose principles are more peculiarly the subject of the first _Critique_. The understanding, according to Kant, is the faculty of rules. Judgment is shown in the application of rules to individual instances. It is the element of individuality and spontaneity in all thinking, for which no rules can be discovered. Judgment cannot be taught, different men possess it in different degrees; it is akin to genius. When, then, Kant turns to examine the faculty of judgment, he is asking whether the mind, in dealing with individuals in all their variety and difference, and in attempting to understand them, is guided by any general rules or principles. The import of this question becomes clear in his relation of it to the familiar question of causation. The principle of causation, as we have seen, is, according to Kant, an a priori principle of the understanding, and is assumed in all experience; but it does not of itself enable us to determine in any particular case what causes what. That is the task of empirical investigation, and needs, as we know, the imagination and insight of the individual investigator; in Kantian language, it is the work of the faculty of judgment. Besides the a priori principle of causation, therefore, we have an indefinite number of empirical causal laws. Kant asks whether the scientist in investigating such laws, and more particularly in considering their relation to one another, is guided by any principles. He finds that the scientist assumes that this indefinite variety is capable of being reduced to some kind of unity, assumes that there is continuity in nature, that knowledge will not remain an aggregate of disconnected rules. Chemistry, for example, has discovered that the overwhelming variety of natural changes can be reduced to the action and interaction of a small number of elements. The chemist proposes to go on and see whether the different elements may not themselves be seen to be forms of one substance.