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Potency, a technical term used principally in philosophy and theology, is the capacity or aptitude in a being to receive some perfection or perform some action. Always a relative term, it means capacity "for" something, e.g., potency to receive energy, to grow, to learn mathematics, to become wise, virtuous. The correlate of potency is act, which expresses the fully present realization or completion of potency. (see potency and act.)

Origins with Aristotle. The general theory of act and potency, with its terminology, originated with aris totle and became the central doctrine of his philosophy (Phys. 187a 12192b 3; Meta. 1045b 271052a 12). For him, potency was necessary to explain change, that is, the transition from one mode of being to another, or the coming-to-be of some new being or mode of being. Change, he argued, requires not only a state of being at its end different from that at the beginning, but also some underlying element or subject that remains throughout the process, thereby forming a bond of continuity between the two terms. This underlying element cannot be identified with either of its changing states or modes of being, since it is now without one, now without the other. It must have the capacity or potency to be now in one state, now in another, so that while in possession of one mode of being it still remains in potency to all other possible states. Thus water that is now actually cold is at the same time potentially hot; an acorn is in potency or has the potency to become an oak tree, and so on.

Every being that is capable of change must therefore be composed of two distinct but mutually interrelated principles called act and potency, the second of which is in act with respect to its present state of perfection and in potency with respect to all other acts within its range of change. In a nonessential or accidental change the potency corresponds to the whole substance or essence of the being, which is thus in potency to its various possible accidental states or modes (see accident). In a substantial or essential change the potency corresponds to ultimate or primary matter, which is in potency to diverse substantial forms (see matter and form).

Thus did Aristotle answer the question that had baffled early Greek philosophers, viz, how is change possible, since new being cannot arise out of pure nothingness, nor, it seems, out of already actualized existing being? His answer was that the new being appearing in change arises out of a relative or partial nonbeing, i.e., out of a preexisting potency that is "not yet" the new being, but has the positive aptitude to become it.

Thomistic Development. Aristotle applied the notion of potency only to the order of change or process. Where no change is possible, according to him, there is no potency but only act. St. thomas aquinas, however, applied this doctrine more widely to explain the intrinsic reason for limited or finite being (De pot. 1.12; C. gent. 1.16,18; 2.5254). He saw potency not merely as a positive aptitude for some attribute, but also as a capacity limiting the amount of perfection a being can possess or receive. Since different limiting capacities can determine different degrees or modes of possessing a common perfection, such as life, knowledge, or power, the notion of limiting potency also explained, for him, how perfection could be multiplied or shared by many different beings, each according to its own potency or receptive capacity (see participation).

Thus St. Thomas explains all degrees of being as different combinations of potency and act. God alone has no potency, either as limiting capacity or as aptitude for change. All beings below Him have at least the composition of the act of existence with the limiting potency of essence, which makes them exist at this particular level, as this particular being. All beings except God have also the act-potency composition of accident and substance, allowing them an individual history of development and change. All material beings, moreover, are composed of substantial form and primary matter.

The notion of limitation was already implicit in Aristotle's theory of potency as receptive capacity, but was never made explicit by him. The reason undoubtedly was that for Aristotle, as for practically all Greeks of the classical period, the finite was identified with the finished, the intelligible, the perfect, i.e., with form or act, and the infinite with the unfinished, the unintelligible, the imperfect, i.e., with matter as pure indeterminate potency. Hence in this perspective the role of limit was associated with act rather than with potency. St. Thomas, on the other hand, synthesized Aristotelian doctrine with the Neoplatonic participation theory developed during the first centuries of the Christian era. According to the latter, any participated perfection has its source in an unlimited plenitude; each subject participating in this perfection shares in it according to its own finite capacity (see neoplatonism; emanationism). St. Thomas identified the infinite source with act and the limiting capacity with potency. Thus the two great currents of Greek metaphysics are synthesized in a basic Thomistic doctrine: "No act is found limited except by a potency" (Comp. theol. 1.18); "Every act inhering in another is terminated by that in which it inheres. Hence, an act that exists in nothing is terminated by nothing" (C. gent. 1.43).

Other scholastic thinkers, like duns scotus and suÁrez, stay closer to Aristotle and incorporate less Neoplatonic doctrine than did St. Thomas. They do not hold or do not stress the role of potency as a necessary limiting principle for finite being, and explain the limitation of act merely by the extrinsic efficient cause or agent that makes the being such as it is.

Meaning and Kinds. The term potency, as understood by St. Thomas, means simply a real capacity for some act. It includes both the positive note of aptitude for act and the negative note of capacity limited to receive only so much act and no more. This potency may, but does not necessarily or always, imply aptitude for change. Because of its essentially relative character potency must be defined in relation to its proportionate act, and hence neither its existence nor its nature can be known except through its fulfillment in actuality.

Active Potency. Potency as aptitude or capacity for act is divided into two main types, active and passive. Active potency is the inner power of an agent to perform some action, though it may not be actually so doing, e.g., the power of an artist to paint a picture, of a bird to fly, or of God to create beings. Since active potency does not of itself imply imperfection, it can be found in both God and creatures. In creatures, however, which now act and now do not, it implies some inner change from inactivity to activity, usually stimulated by an outside agent. When divisions into act and potency are compared, active potency belongs more properly with act than potency; the latter, unless further specified, usually means passive potency. Hence, to avoid confusion in terminology, active potency is more safely referred to as active power.

Passive Potency. Passive potency is the capacity to receive or be enriched with some new perfection not previously possessed, e.g., to acquire new knowledge, to receive heat energy, and so on. This potency alone plays the role of limiting capacity already explained. No passive potency possesses in its own right the perfection or act that it receives; if so, it would itself be the act in question and not a potency for it. Therefore passive potency can be activated only through the action of a cause or causes already possessing, at least in some equivalent way, the perfection that is actualized in the recipient potency. This priority of act over potency is used by Aristotle, St. Thomas, and other scholastic philosophers as a key notion to demonstrate the existence of God as First Unmoved Mover, the pure act required as ultimate cause of change (see motion, first cause of; god, proofs for the existence of).

Pure Potency. The only pure passive potency in the universe, according to Aristotle and St. Thomas, is primary matter. This doctrine has often been grossly misinterpreted to mean that pure potency is a distinct material thing existing on its own, like an atom. Since this potency is of itself pure indetermination, it can exist only as coprinciple in a composite of act and potency that is itself structured or "informed" matter. Hence it can never be isolated or observed by laboratory analysis, and can be grasped only by philosophical analysis as a necessary condition for the properties of beings that can be observed. All complete real beings in the universe, therefore, are either Pure Act or varying combinations of act and potency. Apart from primary matter, all levels and kinds of passive potency are a blend of passive and active potency. Thus, whenever a new perfection is received by any being, this involves some aspect of passivity, or reception, and some aspect of activity, or assimilation of the new perfection into the being of the recipient.

Obediential Potency. Theologians also speak of a special passive potency in creatures as obediential potency. Unlike a natural potency, this does not flow directly from the nature of a thing, but is rather the basic openness or docility of the creature to elevation by God toward some level of perfection beyond its normal fixed status in the hierarchy of being. The principal instances of this type of potency actually given in the present historical order (endless others may be hidden in God's providence for the future) are: (1) the aptitude of rational creatures to be raised to the life of grace and the beatific vision as adopted sons of God; (2) the aptitude of human nature to be assumed into a special hypostatic union with one of the Persons of the Blessed Trinity, as happened in the human nature of Christ in the incarnation, and (3) the aptitude of material things to be made efficacious instruments or channels of divine grace in the Sacraments. [see sacraments (theology of).]

Potency in Modern Philosophy. With the renais sance, the theory of act and potency fell into disrepute along with other Aristotelian and scholastic doctrines (see renaissance philosophy). One of the reasons was that decadent scholasticism often substituted this general metaphysical theory for proper scientific explanations relevant to particular areas of experience. To explain the phenomenon of sleep, for example, by postulating a dormitive potency might be quite correct, but this explanation is so general as to be scientifically sterile. Nor was the theory of potency and act ever intended for such purposes.

Most modern philosophers outside the scholastic tradition have attacked the notion of potency. The notable exception is leibniz (16461716), who attempted to restore it to use in his theory of the monad. A principal reason for this attack is that potency is knowable only through its fulfilling act and is not a complete entity that is discoverable by sense observation or scientific experimentation. Act and potency is the general model of a metaphysical composition of incomplete coprinciples within a complete being; when this notion was lost, the idea of real potency was distorted into that of a physical entity hidden somewhere inside a being, which, if real, should be able to be uncovered by a sufficiently probing scientific analysis. When nothing such was found, as of course it could not be, the notion itself was rejected as medieval superstition. The "clear and distinct ideas" of Cartesian rationalism, the radical empiricism of the English tradition culminating in hume, the conception of nature found in mechanismall had no place for the authentic conception of potency.

Modern philosophies of change, such as that of bergson, make continuous process or succession of states the very essence of reality. Since, for them, no underlying immobile substances exist, they either deny the reality of potency as distinct from act or consider it a mere mental projection of the present into the past.

With the breakdown of mechanistic pictures of the universe in 20th-century science, and the recent return of metaphysics to respectability among philosophers outside the scholastic tradition, the notion of real potentiality is being restored as indispensable for an adequate analysis of reality. This is true not only among philosophers, but even among scientists, most notable of whom is Werner Heisenberg. Indeed it is difficult to see how one can explain any process of organic growth, or the range of unpredictable possibilities of action found in nature from man to subatomic particles, without the help of some such notion.

See Also: possibility; potency and act; matter and form; essence and existence.

Bibliography: a. pompei, Enciclopedia filosofica, 4 v. (Venice-Rome 1957) 3:156264. v. mathieu, ibid. 3:154558. a. mansion, Introduction à la physique aristotélicienne (2d ed. rev. & enl. Louvain 1946). j. stallmach, Dynamis und Energeia (Monographien zur philosophischen Forschung 21; Meisenheim 1959). c. giacon, Atto e potenza (Brescia 1949). r. garrigou-lagrange, Reality: A Synthesis of Thomistic Thought, tr. p. cummins (St. Louis 1950). v. e. smith, The General Science of Nature (Milwaukee 1958). w. n. clarke, "The Limitation of Act by Potency," The New Scholasticism 26 (1952) 167194. j. wild, Introduction to Realistic Philosophy (New York 1948). w. heisenberg, Physics and Philosophy (New York 1958). g. a. de laguna, "Existence and Potentiality," Philosophical Review 60 (195l) 155176. e. b. moore, "Positivism and Potentiality," Journal of Philosophy 48 (1951) 472479.

[w. n. clarke]