41. A principle is that from which anything proceeds. it is also defined as That by which a thing is, is made, or is known. -- From the latter definition it is evident that there are three kinds of principles: principles of composition, those of which a thing is constituted; principles of production, those which concur in the making of things; principles of cognition, those by which we attain to a knowledge of things. The last mentioned are treated of in rational philosophy, and the others in metaphysics; ontology studies the principles of composition or metaphysical principles; cosmology investigates the principles of production or physical principles.
42. Cause is defined as That on which another depends for its being. -- This definition applies both to dependence of reason or order; as, "One proposition depends on another" (but not to mere external connection, as in the proposition, "Day succeeds night"); and to dependence of nature; as, "The fruit depends on the tree." The definition of cause is further explained by that of effect. An effect is that which is produced, or that which passes from non-existence into existence; but that which is not yet existing cannot receive existence except by the action of something else, and to this agent the name of cause is given. Hence two conditions constitute a cause properly so called: (1) its distinction from the effect, and (2) the dependence of the effect upon it; distinction, because that which is ushered into existence cannot be the same as that which has given it existence; dependence, because to be brought by something from non-existence into existence implies a dependence on that thing. Hence it is manifest that principle is more general than cause. For every cause is a principle, but not every principle is a cause: that which proceeds from the principle is very often not produced by it; that which begins with the cause is at the same time produced by it. Principle implies priority of origin only, or, as St. Thomas calls it, "order of origin;" cause implies also priority of time, or, at least, priority of nature, for priority of nature consists in this that one thing depends on the nature of another and proceeds from it.
ART. I. -- POTENTIALITY AND ACTUALITY, OR POWER AND ACT.{1}
43. The first metaphysical principles of every created being, those by which it is constituted, are potentiality and actuality. -- Every created being was, before its existence, in the series of possible beings; it had only a possibility to exist; it was in potentiality. Afterward it existed; then it was in actuality.{2} The possibility to exist and the act of being are, therefore, the two constituent principles of every created being. God alone is not composed of potentiality and actuality. Having always existed, He is pure act.
44. Act{3} is a perfection; a thing is said to be in act when it has its perfection. -- The actuality of a thing is that perfection by which it is not in potentiality but in reality, not in the ideal order but in the ontological order. Hence act gives a new being to the thing, realizes its possibility, fills up its capacity. Therefore, a thing is in act when it has its perfection.
45. Act is pure or not pure according as it excludes all potentiality or is united with potentiality. -- Act may be joined to potentiality in two ways: (1) When it is the act of some potentiality; thus the "soul" by which the body exists is the act of the body; (2) When it is itself in potentiality relatively to an ulterior act; as an "angel," whose nature was at first in potentiality relatively to existence. Act is called pure when it is not joined to potentiality in one of these two ways; otherwise it is not-pure. In the latter case it is called formal act, or act of essence, if it determines the essence to a species of being; as the "form of a plant," which makes it such a species of plant; and act of being or of existence if it is the being or existence itself.
46. Not pure act is divided into first act and second act, and either excludes or supposes another precious act. -- First act is that which does not suppose any other before it, but which prepares the entity for subsequent acts; in this sense the soul is the first act of the body. Second act supposes a first; thus the "operation of a faculty or power" is a second act, because it supposes the existence of that faculty from the first act.
47. First act is divided into subsistent and non-subsistent act according as it can or cannot subsist alone. -- Subsistent first act either subsists alone, as "angel," or can subsist alone, as the "soul of man." Non-subsistent act cannot subsist alone, as the "soul of a brute."
48. Subsistent act is divided into complete and incomplete act according as it has its perfection in itself in such a manner that it cannot be received into any potentiality; or though it can subsist alone, is yet destitute of its perfection if it be without the potentiality in which it ought to exist. -- Complete subsistent act is of such a nature that it cannot be received into any potentiality; as an "angel." Incomplete subsistent act can exist alone, but to have its perfection it requires the potentiality in which it can and ought to exist; as the "human soul."
49. Non-subsistent act is divided into substantial act, which gives being simply to its potentiality, and accidental act, which presupposes being in its potentiality. -- Substantial act gives being simply to its potentiality; such is the "brute soul." Accidental act supposes being already in the potentiality which it informs; as the "whiteness of paper," which supposes the existence of the paper before its whiteness.
50. Potentiality{4} is the aptitude to receive actuality; to be in potentiality is to be apt to receive the act which does not yet exist. -- Potentiality, a metaphysical principle of being, is that by virtue of which a thing, which as yet does not exist, can receive existence from an efficient cause. The word potentiality is here a synonym for the words possibility, capability.
51. Potentiality is logical or real according as it means simply the absolute possibility of existing, or signifies that a being already existing in some way can exist in another way. -- Logical potentiality is only the exclusion of the impossibility of existing, and is so called because it is only in the mind of the cause that is to bring it into existence. Such was the state of all created things before the power of God drew them from nothingness. This possibility or potentiality is also called intrinsic or metaphysical or absolute, to distinguish it from the active power that makes it come into existence. In respect to this active power it is called extrinsic or relative possibility -- extrinsic, because it is to be reduced to act by some one beyond and distinct from itself; relative, because it is referred to the cause that makes the possible entity actual or real. In respect to creatures this relative potentiality is either physical or moral, according as the power which it implies is considered according to the laws of the physical world or according to those to which moral agents are subject. -- Real potentiality supposes the being already existing in a determinate manner; it is therefore the possibility of passing from one mode of existence to another. Metaphysical potentiality, regarded as about to come into existence by the action of some efficient cause, is called objective. To this, subjective potentiality is opposed, which is nothing but real potentiality, and is so called because only a subject already existing can have it. Real or subjective potentiality is pure if it has no act; and of this "primary matter" (materia prima) is the sole example. It is not-pure if it has a beginning of act, but tends to an ulterior act; as a "substance" relatively to its accidents.
52. Potentiality is divided also into proximate potentiality and remote potentiality; the former needs only the aetion of the agent to pass into act, the latter needs other active principles to render this action possible. -- A man with good eyesight is in proximate potentiality with regard to vision, for he needs only the light to see. A blind man is in remote potentiality with regard to vision, because before the light can act on his vision he must be cured of his blindness.
53. An entity is absolutely and intrinsically possible because it implies no contradiction; it is relatively and extrinsically possible because God is omnipotent. -- A thing is not absolutely impossible because it cannot be done by God, but it cannot be done by God because it implies a contradiction or is absolutely impossible. Therefore the possible is so primarily because it implies no contradiction. But if an entity is absolutely possible because it implies no contradiction, it is relatively possible because it can proceed from its cause. And since God is the cause of all being, it is from Him that the relative possibility of every being is derived, and, furthermore, everything that is absolutely or intrinsically possible is also relatively or extrinsically so.
54. From the definition of act and of potentiality this axiom follows: A thing is perfect so far as it is in act, it is imperfect so far as it is only in potentiality. -- Since act is a perfection, that which is in act is perfect, and that which is in potentiality is imperfect. From this axiom is derived the consequent: An act absolutely pure is also absolutely perfect. From the definition of act and of potentiality the following axioms are derived: 1. Being acts inasmuch as it is in act; on the other hand, it is acted upon inasmuch as it is in potentiality; 2. Potentiality cannot of itself reduce itself to act, but requires a being already in act; 3. Every changeable being is composed of act and potentiality. -- For it changes, that is, it begins to be what it was not, or ceases to be what it previously was, only because it had the possibility to be or not to be what it became or ceased to be.{5}
ART. II. -- ESSENCE AND EXISTENCE.
55. In every created being we should distinguish as constituent principles, which are conceived as actuality and potentiality respectively, essence and existence, or the act of existing and that which has this act. -- Every thing that is either is being itself or has being, that is, participates in being. In that which has only participated being we notice, when it is in act, two distinct things: (1) that it is, or the act of being (2) that which it is, or that which has the being. The former is called existence,{6} the latter essence. Essence is also called nature, because it is the source of a being's properties and operations; reason of the thing, because it is the reason why the thing is what it is; firm, because it determines the thing in its species. Yet there are essences which are not pure forms; such are essences composed of matter and form, and in these the form is not the whole essence.
56. In every finite and created being essence is really distinct from existence as a potentiality from its act. -- In God, who is Being by excellence, essence is not distinct from existence but is identical with it. But finite and created beings are composed of essence and existence, as of two principles of being. In fact, no creature has being simply, for this is peculiar to God. All created beings are composed necessarily of potentiality and act. For if essence were not distinct from existence in them, they could be said to exist by their essence. But this is false, for creatures having been drawn from nothing by the creative act of God, have their existence not necessarily and essentially, but accidentally and contingently.{7}
{1} Although Potentiality and Act do not generally receive so full a treatment in text books as is given in the present manual, yet they are of prime importance in Scholastic philosophy, and without them essence and existence, matter and form, soul and body, and the origin of ideas, cannot be understood, while they underlie the principles of moral philosophy.
{2} The term actus originally signified operation; then by extension it came to signify also the principle of operation.
{3} Act is the reduction of a possibility to a reality, of a power active or passive to its complete reality. " -- Harper.
{4} Potentiality is a capability if active, a capability of "doing, acting, energizing, working. . . . Such are the forces of nature, bodily power, the faculties of intellect and will." If passive, it is a capacity of "receiving, of being perfected by another . . . as the capacity of water for receiving the form of heat. 'The Infinite, being most pure Act, has no potentiality; primordial matter being simply and exclusively a passive potentiality, has no act.' " -- Metaphysics of the School, vol. i., p. 585.
{5} There is also an operative power or faculty, such as intellect and will, which may be defined a capacity and aptitude to elicit operations. Power again is distinguished as active or passive. Thus the "vegetative powers" are active because capable of changing that on which they act. "Marble" has a passive power of receiving the form of a statue from an agent. See Psychology, 3.
{6} According to Hume, " whatever we conceive, we conceive to be existent;" therefore our mere thinking gives it existence. He denies the reality of essence, as Locke had done before him. Mill, too, approves the teaching of Locke that essences are merely " the significations of their names." Kant, though aiming to refute the scepticism of Hume, strays even further from certitude, for he denies that we can know things in themselves. The effect of such doctrines on science, morals, and religion, can readily be inferred. See also 96.
{7} See Zigliara, Summa Philosophica, O. 12, v.