An accident, properly so called, is an act having no term of its own, and, therefore, having no metaphysical essence and no possibility of a separate existence. Accordingly, the term of which it is in need must be supplied by a distinct being already existing in nature; and this is called the subject of the accidental act. Hence no accidental act can be conceived to be without a subject.
And here we must reflect that, as the first actuation of an essential term by its essential act has for its result the actual existence of the individual being, so also any second, or accidental, actuation of the term by an accidental act has for its result an actual mode of existing of the same individual being. From this plain truth we infer that a distinction is to be made between accidental acts, which are properly accidents, and accidental modes, which are only accidentalities. An accident, properly speaking, is that which causes the subject to acquire an accidental actuality, and is always an act; whilst the accidental mode is not an act, but an accidental actuality which results in the subject from the reception of the accidental act.
These general notions being admitted, let us inquire into the principles of accidental compounds. An accidental compound is either a compound of substance and accident or a compound of real essence with something superadded. In the first case, “accidental” means the opposite of “substantial”; in the second case, “accidental” [pg 178] means the opposite of “essential.” Thus a falling body is an accidental compound of substance and its momentum, the momentum being a real accident; whereas a man clothed is an accidental compound of individual human nature and dress; the dress being considered as something accidental as compared with the essence of man, though it is a real substance. And in the same manner a mass of gold is an accidental compound of golden molecules, because each molecule fully possesses the essence of gold independently of any other molecule; whence it follows that the addition of other molecules is accidental as compared with the essence of gold, and only increases the quantity without altering the specific nature of gold. Of course, these other molecules are substances, and it is only their concurrence into one mass that is accidental.
It is plain that the constituent principles of an accidental compound are three—viz., the accidental act which entails a modification of the subject; the subject which receives the modifying act; and the accidental mode of being, or the modification, which results from the reception of the act in the subject.
The subject is always a complete physical being, and, therefore, has its own essential act, term, and complement, independently of all things accidental. It becomes the subject of an accidental act by actually receiving it.
The accidental act which is received in the subject must proceed immediately from the action of some natural or supernatural agent. This is evident; for real receptivity is real passivity, and therefore reception is passion. Now, no passion can be admitted without a corresponding action. Hence all accidental act that is properly and truly received in a subject is the immediate product of action, and its production exactly coincides and coextends with its reception.
Lastly, the mode of being which results from the accidental actuation of the subject is only an accidentality, or an accidental actuality, as we have already remarked, and is predicated of the subject, not as something received in it, but as something following from the actual reception of the accidental act. Hence the substance, or the nature, which is the subject of such accidental modes lies under them, not on account of its receptivity, but on account of the resulting potentiality, which is a proper appurtenance, not of the material term, but of the formal complement of the substance. And, in fact, the complement of all created essence always arises from the actuation of a potential term, and therefore is itself necessarily potential—that is, liable to such accidental changes as may result from any new actuation of the essential term. This resulting potentiality is commonly styled mobility, changeableness, or affectibility, and may be called modal potentiality in opposition to the passive potentiality which is the characteristic of the essential term.
Hence a subject is said to receive the accidental act, but not the accidental mode; and, on the contrary, is said to be affected by the accidental mode, but not by the accidental act. We may say, however, that a subject is modified as well by the act as by the mode, because this expression applies equally to the making of the change (mutatio in fieri) and to the state that follows (mutatio in facto esse).
A subject has, therefore, two distinct manners of underlying: the one on account of its receptivity, the other on account of its affectibility; the one by reason of the passive potentiality of its term, the other by reason of the modal potentiality of its complement. Thus a body, according to its passive potentiality, underlies the act produced in it by a motive power, because it passively receives the motive determination, and, according to its modal potentiality, it underlies local movement, this movement being the immediate result of the determination received. And in a similar manner our soul, inasmuch as it is receptive or passive, underlies the act produced or the impression made in it by a cognizable object; and inasmuch as it is affectible, it underlies the feeling or affective movement, which immediately results from the cognition of the object.
We have said that every accident which is received in a subject and inheres in it must be produced by the action of some agent; and this being the case, it follows that the quantity of the mass of a body, and the quantity of its volume, which are not the product of action, cannot be ranked among the accidents received and inhering in the body; and generally all the accidental modes which arise in the subject, in consequence of the reception of accidental acts, are intrinsic modes indeed, but are not received, and do not properly inhere in their subject; they only result in the subject. Moreover, as all such intrinsic modes immediately arise from the intrinsic reception of accidental acts, it follows that those accidental modes which do not arise in this manner must be extrinsic; and therefore such modes, though they are predicated of their subject, do not inhere in the subject, but only in a certain manner adhere to it. All accidental connotations and relativities belong to this last class.