The strictly substantial form contains in itself the whole reason of the being of the substance; for the matter which completes it does not contribute to the constitution of the substance, except as a mere term--that is, by simply receiving existence, and therefore without adding any new entity to the entity of the form. Whence it follows that the form itself contains the whole reason of the resulting essence. “Although the essence of a being,” says S. Thomas, “is neither the form alone nor the matter alone, yet the form alone is, in its own manner, the cause of such an essence.”[31] It cannot, however, be inferred from this that the strictly substantial form is a _physical_ being. Physical beings have a complete essence and an existence of their own; which is not the case with any material form. “Even the forms themselves,” according to S. Thomas, “have no being; it is only the compounds (of matter and form) that have being through them.”[32] And again: “The substantial form itself has no complete essence; for in the definition of the substantial form it is necessary to include that of which it is the form.”[33] It is plain that a being which has no complete essence and no possibility of a separate existence cannot be styled a physical being, but only a metaphysical constituent of the physical being. The schoolmen teach that the substantial forms of bodies _are educed out of the potency of matter_. This proposition is true. For the so-called “substantial” forms of bodies are not strictly substantial, but only essential or natural forms, as they give the first existence, not to the matter of which the bodies are composed, but only to the bodies themselves. Now, all bodies are material compounds of a certain species, and therefore involve distinct material terms bound together by a specific form of composition, without which such a specific compound can have no existence. The specific form of composition is therefore the essential form of a body of a given species; and such is the form that gives _the first being_ to the body. To say that such a form is educed out of the potency of matter is to state an obvious truth, as it is known that the composition of bodies is brought about by the mutual action of the elements of which the bodies are constituted, which action proceeds from the _active_ potency, and actuates the _passive_ potency of the matter of the body, as we shall explain more fully in the sequel.