MECHANICISM. - A philosophical theory, or more generally a current of thought, that interprets the world as a great machine, and seeks the explanation of all sensible phenomena by means of quantity alone, local motion and the notions derived from them (space, figure, time...), thereby excluding any substantial and qualitative diversification and change. M. extends its attempt at explanation also to vital and psychic phenomena, and in this sense it is opposed to vitalism, which recognizes the essential irreducibility of life and of the psyche to mechanical elements alone. Even when restricted to the inorganic world alone, m. has had various formulations in the history of human thought and has been connected with other conceptions of nature, from which, however, it must be carefully distinguished. Often m. has been connected with atomism (v.), insofar as it responds to the same demand for rationalization and visualization of the elements of the physical world. However, not every form of atomism is mechanistic; for example, the atomism of Anaxagoras and Empedocles and the chemical atomism of the 19th century are not mechanistic, since they recognize atoms as endowed with qualities and forces irreducible to one another and distinct from pure local motion. Likewise, not every m. is atomistic, such as Cartesian m., which admits the infinitesimal divisibility of quantity. One must further distinguish m. from mechanics, or the science of motion, experimentally and mathematically founded by Galileo, Newton and Lagrange, even if the founders and practitioners of mechanics were often dominated by more or less mechanistic conceptions.
I. ANCIENT MECHANICISM
The first historical formulation of M. is found in the atomism of the Greeks Leucippus and Democritus (5th-4th centuries B.C.), of Epicurus (4th-3rd centuries) and of the Latin Lucretius (1st century: De rerum natura). In contrast to the doctrines of Epicurus and Lucretius, which place the cause of the movement of atoms in weight, Democritus, according to the fragmentary testimonies that remain, held that no cause should be assigned to the eternal motion of atoms: "Democritus does not think that the cause of things that always are should be sought" (Aristotle, Phys., VIII, 1, 252 a 35). For this reason the mechanistic thought of Democritus has been brought by F. Enriques (Storia del pensiero scientifico, I. Il mondo antico, Bologna 1932, pp. 148-49) close to the concept of inertia, which lies at the basis of the mechanics of Galileo-Newton, according to which uniform rectilinear motion is maintained indefinitely without the intervention of a new force, which would be the cause of acceleration. Certainly the Democritean conception offers more points of contact with the views of modern mechanics than does the later Epicurean doctrine. Nevertheless one must not forget the profound diversity of logical structure between Democritean M. and modern mechanics, as well as between the atomism of the former and the modern atomic theory. For while the modern principle of inertia is the result of the Galilean experimental method, the motion of Democritus's atoms is an a priori assumption, and the coincidence of Democritean M. with the kinetic theory of matter, from Bernoulli to Maxwell, is purely partial and extrinsic. Moreover, the same principle of inertia of modern mechanics, if by many, following Descartes, it has been interpreted in a mechanistic sense with the denial of any reality distinct from the body itself and from its motion, nevertheless in the very founders of mechanics, such as Leonardo and Galileo, and in a non-mechanistic philosophical interpretation, it implies the existence in a body in motion of a reality distinct from the body itself and from its velocity, the impulse or quantity of motion, which fully verifies in itself the Aristotelian notion of variable quality (cf. P. Hoenen, Filosofia della natura inorganica, Brescia 1950, pp. 123-31, 149). Therefore, while the M. of Democritus, as well as all the following forms of philosophical M., is essentially bound to the Eleatic metaphysics of 'identity', modern mechanics is independent of it; so that the acceptance of the latter by no means implies the affirmation of the former.
II. MODERN MECHANICISM
M., with its fundamental idea that all physical phenomena must be explained by elements of figure and motion alone, was taken up again in the modern age, as a reaction to the Aristotelian philosophy of quality, which, especially in the period of the decline of Scholasticism, had sought to explain the new data of experience by recourse to an ever-increasing number of occult qualities. The new and characteristic element of modern M. is the introduction of Galileo's experimental method and, even more, the ever more complete and refined mathematical description of the laws of motion, favored by the developments of mathematics in the 17th century, especially of analytic geometry and infinitesimal analysis. From the philosophical point of view, special mention is due to the M. of Descartes (v.), one of the most rigid and consistent forms, which presents itself as the type of non-atomistic M. in opposition to the atomistic M. of the ancient Greeks, renewed, at the same time as Descartes, by Gassendi (v.).
While rigid M., in consequence of its fundamental conception, denies any active power among bodies and admits no other cause that can influence changes of motion than mechanical impact, following Newton's theory of universal attraction, and then through the development of modern chemistry, more moderate forms of M. established themselves among scientists, which admit the existence of central attractive forces, of affinities and specific properties of the different atoms, irreducible to elements of figure and motion alone. Nevertheless, despite these partial concessions, the mechanistic conception, at least as an ideal tendency, still formed, almost throughout the last century, the premise generally accepted by science. The theoretical demand and the logical consequence of this conception were summed up in famous phrases by Laplace (Théorie analytique des probabilités, Paris 1820, pp. II-III).
The 19th century, however, also presents the rise of an anti-mechanistic current, supported by positivism and energetism (v. DINAMISMO; ENERGIA). The positivists, following Comte (v.) and Mach (v.), level against M. the same reproach that the latter directed at 17th-century Aristotelianism, since it too, in order to explain phenomena, has recourse to hypothetical occult entities, particles and invisible motions, which would be the cause of visible phenomena. Positivist science, on the other hand, wishes to stop at the data of sense, in order to describe the concatenations and constants of phenomena, the laws of nature, without seeking a beyond, without having recourse to metaphysical hypotheses; thermodynamics would offer the example of this. The influence exercised by this philosophy has been great; and in minds which, while accepting its principles, felt the need for a concrete representation, it gave rise to a new epistemological theory midway between positivism and M.: the theory of models, which enjoyed its greatest vogue in England with J. K. Maxwell (Memorie, published in the Philosophical magazine, 1861-62; Treatise on electricity and magnetism, London 1873), W. Thomson (Lectures on molecular dynamics and the wave-theory of light, Baltimore 1884; Popular lectures and addresses, London 1889-94), O. Lodge (Modern views on electricity, ibid. 1889), and was also received in Germany with H. Hertz (Gesammelte Werke, Leipzig 1894-95) and in Italy with A. Garbasso and A. Pastore (Logica formale dedotta dalla considerazione dei modelli meccanici, Turin 1906). The theory of models preserves mechanistic explanation, to the point of affirming with Thomson that a phenomenon is rendered intelligible when a mechanical model of it has been constructed; however, it does not attribute to these models a real ontological value and regards them as a purely mental, subjective schema.
At the beginning of the 20th century, however, the progress of chemistry and of the kinetic molecular theory and especially the experiments on emulsions, carried out by J. Perrin (Les atomes, Paris 1914) for the determination of Avogadro's number and of molecular magnitudes, brought an important contribution to the polemic between positivists and mechanists. The kinetic molecular hypotheses, while not becoming in the strict sense an object of experience, received such confirmations as to place beyond discussion the possibility of ontologically true mechanical representations. The impression on more or less positivist physicists was enormous: the atom and its motions ceased to be convenient fictions and ideal entities, in order to become real entities, a laboratory reality (cf. Bulletin de la soc. franç. de phil., sciences du 27 jan. et 3 mars, 10 [1910], pp. 81-121; H. Poincaré, Dernières pensées, Paris 1913, pp. 196-99; L. Brunschwig, L'expérience humaine et la causalité physique, ibid. 1922, pp. 375-76). This success also benefited the mechanistic philosophical conception, of which Meyerson (v.) became the principal exponent and which was identified by many physicists and philosophers with the kinetic molecular physical theory.
III. CRISIS OF MECHANICISM
Nevertheless, many of the observations already advanced by the energetists, and even more the developments of physics over the last fifty years, have fatally led physicists to revise the fundamental postulates of scientific constructions from the 17th to the 19th century and to renounce, perhaps definitively, a general interpretation of the mechanical order. Impact, with the elastic forces that are aroused in it, central attractive and repulsive forces, electromagnetic forces depending at once on charge, distance and velocity, and acting perpendicularly to the line joining the centers, radiant energy and gravitational and electromagnetic fields, had already manifested themselves as phenomena irreconcilable with the fundamental position of M., requiring the acceptance of real elements distinct from pure extension and local motion and thus necessarily introducing the Aristotelian notion of variable quality, as has already been observed apropos of impulse (cf. Hoenen, op. cit., pp. 149-60).
More fundamental is the theoretical crisis permanent in the very essence of M., which has been brought to light by Meyerson himself. M., yielding to the demand for visualization of the real, limits rationality to the identical and the permanent; but a rationalization of the world understood in this sense fatally leads, if one wishes to escape Eleatic monism, to the negation of rationality itself. Just as Democritus had recourse to the affirmation of the reality of nothingness, so too Meyerson is forced to admit the existence of the irrational. There is something that resists rationalization as understood by M.: thought, sensation, energy; local motion itself presupposes forces that act by causing its variation and its transmission from one body to another; even the simple numerical multiplicity of the single fundamental matter presupposes a heterogeneity inexplicable in the face of the demand of mechanistic rationality. Hence Meyerson's conclusion, which is the logical conclusion of every M.: science is born from the clash between the rational and the irrational.
Finally, the theories of relativity and of quanta have brought new difficulties to the mechanistic interpretation, with the relativity of measurements of space and time, the variability of mass, the conversion of mass into energy, the quantization of elementary phenomena, the dualism between corpuscle and wave, the uncertainty principle, the new statistics of Fermi and Bose with the indistinguishability of corpuscles, etc. The difficulties are indeed such that many physicists believe they must conclude to the exclusion not only of a mechanistic representation, but of any objective representation of elementary realities, returning to the positivist conception of science. On the other hand, however, physicists in describing reality spontaneously make use of purely Aristotelian terminology, such as energy and potential states, the distinction of the true atom as a unitary totality from the pure dynamical aggregate of electron and proton (line spectrum and continuous spectrum), the virtual presence of electrons and mesons in the nucleus, transformations of protons into neutrons and vice versa not by simple aggregation or disintegration of identical permanent elements, generation and destruction of pairs of electrons. All this gives a glimpse of the possibility that science may attain a vision of the physical world and of its elements that is objective but not mechanistic, in accordance therefore with Aristotelian-Thomist epistemology and cosmology.