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Any concrete idea implies a group—it may be an indefinitely numerous group—of propositions, whose predicates express different qualities and relations of one and the same subject. Partly by common observation of actual instances, and partly by common contemplation of instances or types such as all ordinary conversation involves, people who speak the same language learn to apply the same concrete-general name to all entities of a given class. At the same time they learn to imply certain properties as essentially connected with the class in question; so that anything not possessing an essential property is not a thing of the given sort. This insures that the analytic part of knowledge shall consist in necessary judgments. The necessity rests logically on the human agreement as to the meaning of names, but this agreement, in its turn, rests on the uniformity of human experience derived from the things named. There are, however, properties, not obviously connoted of a class-name, which may be found to belong to the whole of the class, whether theoretically, by deduction from judgments which are analytically certain, or practically, by the complete absence from experience of instances of the given class not having the given property.

While the joining of various predicates to a given subject, and the concurrent grouping of concrete subjects into allied species and genera, gives rise to what may be called descriptive knowledge, explanatory knowledge consists in applying an understood predicate (usually involving causal relationship) to a variety of concrete things or classes, whose community of nature, origin, or purpose thereby becomes apparent. Natural laws evidently involve abstract ideas predicates applicable to all individual subjects of a given type, which may of course include many subsidiary types. Such predicates signify either the mode in which one class of entities is causally related to another class of entities (e. g., flowering plants to the insects which are necessary to fecundate them) or that in which one quality of a class is causally related to another quality (e.g., expansion of a material to the rise, and contraction to the fall, of its temperature).