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.