ENERGIA. — In general, it signifies activity, action, power, strength. In its technical sense, the concept, specifically defined by science, appears only recently.
Aristotle, in Met., IX, 8, 1050 a 22, determines the notion of ἐνέργεια in relation to ἐντελέχεια. Ἐνέργεια (ἐν ἔργῳ εἶναι) indicates activity or actualization, whereas ἐντελέχεια (ἐν τέλει ἔχεια) indicates the actuality or perfection that results from activity, that is, the form: τοῦτο ἐνέργεια λέγεται κατὰ τὸ ἔργον καὶ συντείνει πρὸς τὴν ἐντελέχειαν (Met., IX, 8, 1050 a 22; cf. s. Tommaso in lume loc.). Nevertheless (H. Bonitz, Index Aristotelicus, Berlin 1870, 253 b, 46 ff.), Aristotle generally uses ἐνέργεια and ἐντελέχεια as synonyms, to indicate act or perfection (cf. W. D. Ross, Aristotle's metaphysics, II, Oxford 1948, p. 245). Ἐνέργεια, act (also generally said of motion, κίνησις; nevertheless, in Met., IX, 6, 1048 b 18-36, κίνησις is opposed to ἐνέργεια, as imperfect act to immanent, perfect activity), is moreover conceived by Aristotle in opposition to δύναμις, which is capacity, power, in order to explain change (Phys., I, 8, 191 b 28; cf. Met., IX, 6 ff.): every transition takes place from δύναμις to ἐνέργεια: μεταβάλει πᾶν ἐκ τοῦ δυνάμει ὕντος τὸν τὸ ἐνέργεια ὄν. (op. cit., XII, 2, 1069 b 15). Ἐνέργεια, however, is in itself prior to δύναμις: φανερὸν ὅτι πρότερον ἐνέργεια δυνάμεις ἔστιν (op. cit., IX, 8, 1049 b 5). Since matter is ὕλην ἂ δυνάμεις (De An., II, 1, 412 a 9), indeed pure δύναμις (Met., VII, 3, 1029 a 20), it is opposed to ἐνέργεια (op. cit., VII, 13, 1038 b 6). In medieval Aristotelianism, ἐνέργεια corresponds to act, δύναμις to potency, and ὕλη to matter.
The form of e. that most immediately strikes experience is the e. of motion, whereby a body in motion possesses the capacity to act upon other bodies by striking them. Aristotle, according to the principle “everything that moves is moved by another” (Phys., VIII, 4, 256 a 3), thinks that this capacity resides in the mover and, when the latter no longer touches the body moved, in the air, which has received from the mover the virtuality of continuing to push the body (cf. Phys., VIII, 10, 267 a 18 ff.). According to Giovanni Filopono (Phys., IV, 8; ed. Vitelli, Berlin 1888, p. 636), a Christian commentator (6th cent.), this virtuality resides instead in the body itself, although it is caused by the mover in the initial impulse. In the Middle Ages, Giovanni Buridano, also basing himself on experimental observations, took up Giovanni Filopono’s idea with his theory of impetus: that virtuality called “impetus,” which resides in the moving body and maintains its motion, and upon which depends the activity it displays when encountering other bodies, is a quality that always remains unchanged unless destroyed externally, and therefore continues to move indefinitely (the first outline of the law of inertia). Buridano makes a reference to the quantitative magnitude of impetus: “The more matter there is, the more that body can receive of that impetus, and the more intensely, just as iron can receive more heat than wood or water of the same quantity” (Phys., VIII, q. 12). Elsewhere: “The mover impresses upon the moved not only motion but also, generally, a certain impetus... And the faster the motion, the more intense that impetus also becomes” (De caelo, III, q. 2). The magnitude of impetus is therefore directly proportional to the quantity of matter and to the velocity of the moving body.
Descartes attributes to every body in motion a quantity of motion, directly proportional to its mass and velocity (= mv; Princ. Phil., II, n. 36; ed. C. Adam and P. Tannery, Paris 1897–1910, VIII, p. 61); bodies act upon one another only through mechanical impact, by which they transmit a determinate quantity of motion. The total quantity of motion, placed into action by the Creator at the beginning of the world, remains identical: “I hold that there is a certain quantity of motion in all created matter, which never increases or diminishes; and that, when one body makes another move, it loses as much of its motion as it gives to it” (Correspondence, ed. cit., V, p. 135). This is the first step toward the law of the constancy of e.
Leibniz observes that Descartes committed an error, error memorabilii Cartesii (ed. L. Dutens, III, Geneva 1768, p. 180 ff.), when he asserts that the quantity of motion remains constant. What remains constant, for Leibniz, is the quantité de force: “Thus it is not the same quantity of motion that is preserved, but the same quantity of force, which must be assessed by the effect it can produce” (ed. cit., III, p. 201). This quantity of force is called by Leibniz “living force” (ibid., p. 318), a name that has remained in use to the present day; he also demonstrates that “living force” is proportional to mv² (ibid., p. 181). Leibniz thus asserts the law of the constancy of e.; he knows, however, only mechanical e. This is understandable if one recalls that, for Leibniz, mechanism prevails in the physical world and every activity occurs through mechanical impact.
During the 18th cent., the conception of heat as a true substance, as a fluid passing from one body to another, prevailed. But at the beginning of the 19th cent., B. T. Rumford and H. Davis demonstrated the transformation of the e. of motion into heat. It followed that the law of the conservation of living force is not exact, since living force can instead be transformed into heat. This immediately leads to the modern principle of the conservation of e. Leonardo da Vinci had already realized that in machines the work of resistance equaled that of power. And Galileo had clearly stated that machines cannot create e. The principle of the conservation of e. in its modern sense is due to J. R. Mayer, L. A. Golding, J. P. Joule, and H. von Helmholtz. The first to speak of it was J. R. Mayer, in a publication of 1842 (J. R. Mayer, Bemerkungen über die Kräfte der unbeliebten Natur, in Liebig's Annales der Chemie, 42 [1842]). He asserts that there exists in nature a single force, which always remains the same amid the continual changes of the organic and inorganic world; and he derives this principle from causality: “Forces are causes and consequently the principle ‘causa aequat effectum’ can be fully applied to them” (loc. cit., p. 233). From this he deduces that forces are “indestructible, changeable, imponderable objects” (loc. cit., p. 234). J. P. Joule determined experimentally the numerical relation between mechanical e. and heat; he, however, and likewise A. Golding, was convinced of the principle for a priori reasons. H. von Helmholtz, too, appealed to the principle of causality in establishing the principle of the conservation of e.; in particular, he demonstrated it on the basis of the impossibility of perpetual motion. Moreover, through his purely mechanical interpretation of the principle, he helped to strengthen II.
In 1824 Sadi Carnot, and later R. Clausius, discovered the second law of thermodynamics, by means of which the entropy of a system can be defined (v. TERSIOLOGIA). It is demonstrated that within a thermally isolated system, in all irreversible transformations—that is, in fact, in all real transformations—entropy increases. This means that the thermal e. of the system degrades, that is, becomes leveled out. And since every e. can be transformed into heat, all the e. of the system becomes leveled out in the course of its transformations. This leads to a general leveling of e., which tends to become uniformly distributed throughout the universe in the form of heat. From the law of entropy it follows that the world evolves toward a state of maximum entropy, in which all e. will have been transformed into heat and the temperatures of bodies will be equal. The statistical conception of entropy somewhat moderates this conclusion.
By taking a position in the purely physical field against the prevailing mechanistic doctrine, G. Ostwald strayed into philosophy, going so far as to construct, with the concept of e., a metaphysical system, energetics. Energetics first of all eliminates matter: «Le caractère distinctif de l'énergétique est l'abandon du dualisme qui a regné jusqu'ici entre la matière et l'énergie; celle-ci prend la place du concept le plus général» (G. Ostwald, L'évolution d'une science, la chimie, Paris 1910, p. 313). «En analysant la matière, en déterminant les parties composantes, nous sommes arrivés à voir qu'elle constitue une notion superflue» (G. Ostwald, L'énergie, ibid. 1910, p. 171). Matter is in fact a complex of three properties: extension, weight, and mass. These properties are three factors of e., whose combination is indispensable if the things of this world are to constitute an object of experience. Thus every body is a complex of e. (G. Ostwald, Vorlesungen über Naturphilosophie, Leipzig 1905, pp. 170, 171, 180, 181). Only e. exists, and it is the true, unique reality: «Die Energie ist die allgemeinste Substanz, denn sie ist das Vorhandene in Zeit und Raum, und sie ist das allgemeine Accident, denn sie ist das Unterschiedliche in Zeit und Raum» (G. Ostwald, op. cit., pp. 146-47). We know the world by means of sensations, which are differences of e. (cf. G. Ostwald, La déroute de l'atomisme contemporain, in Revue générale des sciences, 6 [1896], p. 950). Every phenomenon is a displacement of e. in space or a change in the form of e. «On entend par énergétique le développement de cette idée que tous les phénomènes de la nature doivent être conçus et représentés comme des opérations effectuées sur les diverses énergies» (G. Ostwald, L'énergie, p. 119). Energetic processes are also important in psychic, social, and cultural life. Ostwald even formulated the «energetic imperative»: «Vergunde keine Energie, verwerte sie!» (G. Ostwald, Der energetische Imperativ, Leipzig 1912, passim).
In physics, e. in general is that which can be produced by work or converted into work, work being the application of a force over a displacement (dL=1/2s). The e. of a body is its capacity to perform work: a compressed spring, a body in motion, a fuel, etc., possess e. E. may exist in very different forms: mechanical, electromagnetic, luminous, gravitational, elastic, chemical, etc. Any activity between bodies takes place by means of their e. E. is therefore a concept of very broad significance in physics.
In mechanics, kinetic and potential e. are considered. Kinetic e., also called living force, is that possessed by a body in motion and which it can develop when coming to rest, measured by the formula frac{1}{2} mv².
Potential or positional e. is that which a body possesses by virtue of being in a given position in a field of forces, insofar as, under the action of these forces, it generally has the possibility of performing work. In mechanics, the principle holds that in a conservative field of forces the sum of the kinetic and potential e. of a body is constant. This is the principle of the conservation of e. in mechanics, and it essentially indicates that the work performed by the body depends on the endpoints of the trajectory. In practice, however, this principle is never verified; for example, once set in motion, a pendulum should never stop; this does not happen because of friction. At the beginning of the nineteenth century, through Joule’s experiments, it became possible to connect mechanical work L with heat Q by means of the relation JQ=L; J is the coefficient connecting heat and work, the mechanical equivalent of heat; when work is measured in kilogram-metres and heat in gram-calories, J=427; that is, one calorie, when transformed into work, produces 427 kilogram-metres, and conversely the mechanical production of one calorie requires the loss of 427 kilogram-metres of work. It should be observed that it is always possible to transform work into heat; but, according to the second law of thermodynamics, heat cannot be transformed into work at will. Having recognized the transformation of work into heat and vice versa, one is led to admit the possibility of transforming any form of e. into any other form, according to precise relations. It is therefore possible to extend the principle of the conservation of e., valid in mechanics, to every other form of e.; for all phenomena, the e. of an isolated system—that is, one not subject to external forces or actions—remains constant. This is the first law of thermodynamics, and it has immense importance in physics, like the analogous principle of the conservation of matter and mass. The principle of the conservation of e. is based less on experience than on our theoretical conviction of the constancy of nature. Thus, for example, H. Poincaré (La science et l'hypothèse, Paris 1902, p. 153), E. Meyerson (Identité et réalité, 4e ed., ibid. 1932, p. 234), and others maintain.
With the theory of relativity, the concept of e. received new clarifications. Toward the end of the previous century, it had been observed that, when electrons and positive ions are in motion, the force required to give them the same acceleration is greater the faster their motion, and therefore the greater their kinetic e., as though the increased kinetic e. had increased their mass. With the theory of relativity (v. RELATIVISMO), A. Einstein found that as the velocity of a body increases, its mass also increases. He calculated that the increase in mass corresponds to the increment of kinetic e. divided by the square of the velocity of light in a vacuum, according to the formula Δ m = frac{Δ z}{c²}. As a consequence, Einstein asked himself whether, just as kinetic e. has a mass, it should not be thought that the entire mass of the body was e.: matter would be an immense concentration of e., according to the formula: E=mc². Not everyone accepted these views in the same way; many accepted Einstein’s relation only for the mass of kinetic e., not for the total mass of the body.
After the theory of relativity, research in modern atomic physics developed, and it was found that a positive and a negative electron, on meeting, give rise to a photon whose e. is equivalent to that of the two destroyed corpuscles. It also follows that the inverse phenomenon is possible. Einstein’s formula is likewise verified in nuclear reactions, in which the e. involved corresponds to the variation in the mass of the reacting bodies, according to Einstein’s formula. On the basis of these relativistic developments, the principle of the conservation of e. is not true, nor does that of the conservation of matter remain true. Therefore, the two distinct principles must be absorbed into the more general principle of the conservation of momentum; since momentum belongs both to matter and to e., it must be conserved through its various forms.
Just as with matter, so too with regard to e. the continuous conception gave way to the discrete one. Until M. Planck, it was thought that e. entered into phenomena through continuous variations. In 1900, while studying the distribution of e. in the spectrum of the «black body», Planck supposed that e. was emitted and absorbed non-continuously, according to exact multiples of an elementary quantity e called the «quantum», linked to the frequency y of the radiation by the formula z = hc, where h is Planck’s constant (= 6,6 to 10⁻²⁷ in eq.s.). This constant is an action (e. multiplied by time) which Planck rightly declared to be not a purely mathematical abstraction but a true physical reality. A few years later, in 1905, while explaining the photoelectric effect, Einstein supposed that radiant e. travels through space quantized in quanta of light, or photons. Thus Planck’s theory, according to which e. is absorbed and emitted in quanta, is integrated with Einstein’s, which states that e. travels through space in quantized form.
This conception was subsequently confirmed and taken up again in quantum mechanics, which, observing that radiant e., that is, photons, displays wave phenomena, e.g., the diffraction and interference of light, and corpuscular phenomena, e.g., the photoelectric cell and the Compton effect, deduces, in agreement with W. Heisenberg’s uncertainty principle, that radiant e. has neither a solely corpuscular nor a solely wave-like character, but that, according to N. Bohr’s principle of complementarity, the corpuscular and wave-like aspects are «like two faces of an object, which cannot be observed together and which nevertheless must both be examined one at a time in order to describe the object completely» (L. De Broglie, I quanti e la fisica moderna, Italian translation by U. Richard, Turin 1938, p. 228; cf. E. Persico, I fondamenti della meccanica atomica, Bologna 1945, p. 142 ff.).
As regards the relationship between matter and e., in view of the experimental results, it must be said that e. manifests a property similar to that of matter, called mass, by which it resists the push of a force. This in no way implies that every mass must be a concentration of e.
From a metaphysical point of view, e. is the foundation of the activity of bodies, since every corporeal operation takes place by means of e. Every being, by virtue of its intrinsic finality, has an essential tendency toward the fulfillment of itself; this fulfillment takes place through operation, which brings it into communication with other realities. «Ἐκαστὸν ἔστιν, ὦν ἔστιν ἔργον, ἔκαστὸν τοῦ ἔργον (De caelo, II, 3, 286 a 8).» Aristoteles hic loquitur, dicens quod unumquodque quod habet propriam operationem, est propter suam operationem; quaelibet enim res appetit suam perfectionem sicut suum finem, operatio autem est ultima rei perfectio» (s. Tommaso, De Caelo, II, lectio 4). This also applies to the body, from whose essence the operation proceeds. E., which grounds the activity of bodies, is therefore rooted in the very essence of the body as an extension of II. E. is not matter, as enregranno maintained, but a value essential to the structure and behavior of matter.
E. is studied and measured by physics. The quantitative measurement of e. is possible because it exists in the extended; the concrete extended, insofar as it is extended, is measurable and subject to the laws of geometry and mathematics. Because of this, e. too is measurable and subject to calculation (cf. P. Hoenen, Cosmologia, Rome 1945, p. 176 ff.).
BIBL. For the historical and conceptual aspects, besides the works already cited, V. K. Lasswitz, Geschichte der Atomistik vom Mittelalter bis Newton, Amburg 1889–90; Rosenberg, Geschichte der Physik, Brunswick 1884; P. Duhem, Étude sur Léonard de Vinci: III. Les précurseurs parisiens de Galilée, Paris 1913; id., Le système du monde, there 1913–17; D. Nys, Cosmologie, I, Louvain 1928, p. 261 ff.; E. Meyerson, Identité et réalité, Paris 1932; A. Maier, Die Impetiatheorie der Scholastik, Vienna 1940; F. Stitzer, Energie. Eine Darstellung des Energie-Bezirkes, Munich 1949. For the physical-mathematical development: for classical physics cf., among others, E. Perruca, Fisica generale e sperimentale, Turin 1941; for the theory of relativity, M. von Laue, Die Relativitätstheorie, Brunswick 1921; for quantum mechanics, besides the classic treatises of L. De Broglie, Introduction à l'étude de la mécanique ondulatoire, Paris 1930; W. Heisenberg, Die physikalischen Prinzipien der Quantentheorie, Leipzig 1930; and E. Schrödinger, Abhandlungen zur Wellenmechanik, there 1928, the clear treatment by E. Persico, I fondamenti della meccanica atomica, Bologna 1945.
#### ENERGUMENI: V. OSSESSIONE E OSSESSI.