Every one who is acquainted with metaphysical matters will acknowledge that it is of extreme importance that these terms and others of a like nature, which are continually employed in metaphysical analysis, be clearly understood by all students of philosophy. So long as our language has no definite words by which to designate the essential constituents of things, no hope can be entertained of advancing the interests of metaphysics by means of vernacular books.
Act and potency, in material things, are called form and matter respectively; hence, material substance is said to consist essentially of matter and form. The forms of natural things are usually divided into substantial and accidental. The substantial form is commonly defined as that which gives the first existence to its matter—quæ dat materiæ primum esse, or simpliciter esse. It is sometimes defined, also, as that which gives the first existence to a thing—quæ dat primum esse rei. But this second definition is open to misconstruction; because, when the thing in question is a physical compound having a number of material parts, the form that gives to it—that is, to the compound essence—its first existence is its physical composition, which is not a substantial, but an essential, form, as we shall see presently.
The accidental form is defined as that which gives an accidental mode of being—quæ dat esse secundum quid. This definition is universally admitted; but it is a remarkable fact that the examples of accidental forms given by most philosophers do not support it. Thus, the form of a statue and the form of a column are not forms giving to the marble any accidental mode of being, but are the very modes of being, which have resulted in the marble from the reception of suitable accidental acts. Therefore, what is called the form of a statue is not a form giving a mode of being, but the mode itself, on account of which we give to the marble the name of a statue. Suarez and others have indeed pointed out the necessity of distinguishing the forms dantes esse from the forms dantes denominationem; yet, even to this day, in our philosophical treatises, the definition of the former is almost exclusively illustrated by examples of the latter. True forms are acts, whilst modes of being are actualities; and therefore modes of being should not be called forms, but formalities. As, however, the word form is in general use in this last sense also, the best thing we can do is to retain the term, and add to it a suitable epithet. I would call them resultant forms, or consequential forms; and in the same manner, when actuality is styled act, I would call it consequential act, or complementary act, that it may not be confounded with act proper.
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.
The essential form is that which gives to a thing its specific nature. This definition coincides with that of the substantial form whenever the specific nature of which we treat is physically simple—that is, without composition of material parts—for, in fact, such a simple nature receives its species from the same form that gives the first being to its matter. Hence, the essential form and the substantial form are one and the same thing so long as there is question of simple or primitive beings. But the definition of the essential form is no longer equivalent to that of the substantial form when the specific nature constituted by it is physically compounded of material parts; because such a compound nature receives its species from its specific composition, which is not a substantial form, though it is essential to the specific compound.
The accidental form is that which gives to its subject an accidental mode of being, or an esse secundum quid, according to the language of the schools.
The so-called resultant form is the actuality resulting from the position of any true form. As, therefore, true forms are either substantial, essential, or accidental, so, also, are all the resultant forms. From the substantial form results the actuality of the primitive being, which, as primitive, is always free from material composition; from the essential form results the actuality of every specific nature, which involves composition of material parts; and from the accidental form results the actual modification of the subject in which it is received.
I have dwelt purposely on these considerations, because the word form, and its derivatives, formal, formally, formality, etc., are variously employed, and sometimes loosely, in philosophy, and because, without a clear and distinct notion of the different kinds of forms, many fundamental questions of metaphysics cannot be rightly understood. I might say nearly as much respecting the word matter, which is the metaphysical correlative of form; but it will suffice to remark that matter, in philosophy, always means a receptive potency which is actuated by a form; so that, if the form is accidental, the word matter stands for material substance itself as receptive, because it is the substance that receives accidental forms; if the form is essential in the sense above explained, then the word matter means the totality of the material parts required for the constitution of any given specific compound, including their actual disposition to receive the form in question; and if the form is substantial, then the word matter expresses only one of the constituent principles of primitive material substance—that is, the potential term of substance; which is first actuated by such a form.