It is also necessary to make a well-marked distinction between _substantial_ and _essential_ forms. The necessity of this distinction is sufficiently shown by the very existence of the two scholastic definitions of form. In fact, two definitions imply two concepts. The first definition, _Forma est id quod dat primum esse materiæ_, strictly belongs to the substantial form, as every one knows; but the second, _Forma est id quod dat primum esse rei_, is more general, and extends to all essential forms, be they substantial or not. Thus, we can say that velocity is the essential form of movement, though, of course, it is not a substantial form, as movement is not a substance. The same distinction is to be admitted with regard to natural compounds, at least in the opinion of those philosophers who oppose the Aristotelic theory of substantial generations, or teach that bodies are made up of primitive, unextended elements. Indeed, if chemical combination does not destroy the essence of the combining substances, it is obvious that the compound substance which arises out of the combination will have no _special_ form, except the combination itself; and such a form, however essential to the compound substance, cannot be a substantial form in the sense of the Peripatetics; because it gives existence to the compound nature only, and not to its matter. Again, if the molecule of a primitive body, as hydrogen, is nothing more than a system of material points or elements connected with one another by dynamical ties, and subject to a law of vibratory movement, which allows the molecule to contract and dilate, then it is evident that the essential form of such a molecule will be its specific composition; for the composition is the immediate constituent of all material compound. Accordingly, since the scientific views which lead to these conclusions are widely received, and very well founded on chemical and other data, and can be philosophically established by the very principles of ancient metaphysics, the said distinction between substantial and essential forms is to be acknowledged as a very important one in questions connected with modern science. Lastly, essential forms are to be admitted, not only in natural, but also in artificial and in moral, compounds. A clock has its essential form, without which it would cease to be a clock; a family has its essential form, without which it would cease to be a family; and yet it would be ridiculous to talk of a clock or a family as having a substantial form. It is, therefore, necessary to divide all true forms into _substantial_, _essential_, and _accidental_, and to place in a separate class all the so-called _resultant_ forms above mentioned. Thus, the _substantial_ form is that which gives the first being to matter. This definition comes from Aristotle himself, and has been universally received by all metaphysicians.