PROLEGOMENA TO ONTOLOGY

§. 9.

Why Ontology is of the widest use.

Since ontological definitions and propositions can be applied to any beings whatsoever, whether in their general state or in some specific given state (§. 8); the utility of Ontology manifests itself everywhere.

And it is for this reason that, whenever we continue the demonstration of principles until they become manifest a priori Wolff uses "a priori" to mean knowledge derived from the very nature or definition of a thing, rather than from sensory experience alone., we always eventually fall back upon the principles of first philosophy. Consequently, without these principles, demonstrative knowledge can in no way be obtained in other disciplines. Mathematics original: "Mathesis" itself speaks to assert this truth. For Euclid resolves his demonstrations into ontological principles, which he assumes as axioms without proof—such as that the whole is equal to all its parts taken together, that the whole is greater than any of its parts, and that things equal to the same third thing are equal to each other.

Now, even if the principles that pure Mathematics borrows from Ontology are so manifest (if you look at the Euclidean Elements) that they can be taken without proof, and the notions borrowed from it—such as equality, majority, minority, and congruence—are so clear that even a confused understanding is sufficient; nevertheless, the same does not hold in the philosophical disciplines. These disciplines also borrow notions from Ontology, but there, confused notions do not suffice; instead, they easily lead one into error. Furthermore, these disciplines take principles from Ontology which are by no means acknowledged as true without proof. Indeed, not even Mathematics can always safely do without distinct notions and demonstrated ontological principles. Illustrious examples of this will occur in the treatment itself. It will suffice here to have appealed to the notion of similarity and the principles depending on it, which I have shown in the Elements of Universal Mathematics original title: "Elementa Matheseos universæ" to have no contemptible use in Geometry. It will also appear from what follows how great the utility of Ontology is in everyday life, and how much rashness in judging arises from a deficiency of ontological notions.

§. 10.

What the meaning of terms used in common speech ought to be.

If any ontological terms are used in common speech, the commonly accepted meaning is to be attributed to them, though it must be rendered determined and fixed. For in all philosophy (§. 142 Preliminary Discourse), and consequently also in Ontology, which is a part of it (§. 73 Preliminary Discourse), one must not depart from the accepted meaning of words. Therefore, for ontological terms that are also used in common speech, no other meaning should be attributed than that which is commonly attributed to them.