_Substantial form._--Coming now to the notion of the substantial form the first thing which deserves special notice is the fact that the phrase “substantial form” can be interpreted in two manners, owing to the double meaning attached to the epithet “substantial.” All the forms which supervene to a specific nature already constituted have been called “accidental,” and all the forms which enter into the constitution of a specific nature have been called “substantial.” But as the accident can be contrasted with the _essence_ no less than with the _substance_ of a thing, so the substantial form can be defined either as that which gives the first being to a certain _essence_, or as that which gives the first being to a _substance_ as such. The schoolmen, in fact, left us two definitions of their substantial forms, of which the first is: “The substantial form is that which gives the first being to the matter”; the second is: “The substantial form is that which gives the first being to a thing.” The first definition belongs to the form strictly substantial, for the result of the first actuation of matter is a primitive substance; whereas the second has a much wider range, because all things which involve material composition, in their specific nature, receive the first specific being by a form which needs not give the first existence to their material components, and which, therefore, is not strictly a substantial form. Thus a molecule of oxygen, because it contains a definite number of primitive elements, cannot be formally constituted in its specific nature, except by a specific composition; and such a composition is an essential, though not a truly substantial, form of the compound, as we shall more fully explain in another article.