QUALITY. — Q. (ποιότης) means, in a generic sense, mode of being. The universe, material and spiritual, not only presents a multiplicity of beings (v. PLURALISM), but also diverse and changing aspects, determinations, and perfections (colors, sounds, weight, hardness..., thoughts, affections, virtues...), to which the generic term q. may be applied.
In opposition to the atomists, who restricted themselves to considering quantitative elements, Anaxagoras (v.) and Empedocles (v.) introduced the concept of q. into the explanation of phenomena. Plato treats q. as a fundamental idea (Theaet., 182 ff.); Aristotle considers it the third of the supreme genera of being
(Met., V, 7, 1017 a 25-30; I, 8, 8 b 25). St. Thomas (Sum. Theol., 1ª, q. 28, a. 2; 1ª-2ª, q. 49, a. 2) defines q. as «modus essendi», «dispositio substantiae» and distinguishes essential q., the determination form (v.), from accidental q., or q. properly so called, a further determination of substance, really distinct from it (v. ACCIDENT; SUBSTANCE). Accidental q. is classified according to a fourfold distinction: 1) as a direct determination of substance: habits (v.) and dispositions (knowledge, virtue, vice, health, illness, supernatural virtue, Grace...); 2) as the principle of operation or activity: operative powers or faculties (senses, intellect, will...); 3) as the principle of alteration or passivity: sensible q.; 4) as determinations of quantity: form and figure. Descartes (v.) defines q. as a property of substance; it is twofold, thought, or q. of the spirit, and quantity, or q. of the body (Princ. philos., I, n. 56 and 63; ed. Ch. Adam-P. Tannery, VIII, 26 ff.). In Kant, q. is one of the modes of judgment to which the categories of reality, negation, and limitation belong (Critique of Pure Reason, Italian trans. by G. Gentile and G. Lombardo-Radice, I, Bari 1919, p. 109 ff.). Hegel posits q. as a moment in idea (v.); it is opposed to indeterminate being as its first and immediate determination and is divided again into pure being, determinate being, and being-for-itself (The Science of Logic, Italian trans. by A. Moni, I, Bari 1924, p. 72 ff.). In Bergson (L'évolution créatrice, Paris 1940, p. 300 ff.) q. is a stable perceptual vision of change and real becoming, Boutroux (v.) asserts the qualitative element as essential to all reality (De la contingence des lois de la nature, Paris 1902, p. 29 ff.).
Sensible q. — In philosophy, the problem of q. refers particularly to sensible q. Indian thought had already affirmed their subjectivity; among the Greeks, first the Eleatics and then the atomists regarded them as deceptive and subjective appearances; Protagoras and Aristippus highlighted their relativity; Plato declared them objects of sensation and not of science (Theaet., 186). Aristotle (De an., II, 6-12) and subsequently scholasticism defended the objectivity of sensible q., distinguished into «primary» (hot and cold, dry and moist) and «secondary» or derivative qualities. During the scholasticism (v.), there was considerable discussion of the «qualitates occultae», postulated to explain natural phenomena and forces: GALILEA (v.), Des-
cartes, LOCKE, JOHN (v.) took a position against them and against all sensible q. Mechanicism (v.), atomism (v.), denies the existence of q. in the physical world, reducing all material reality to quantity alone and local motion, and regarding qualitative varieties as purely subjective reactions of the senses. The problem has a properly critical-epistemological aspect (for which V. PERCEPTIONISM) and a physical-cosmological aspect, consisting in determining the nature and properties of the various sensible q. and the laws governing them; it is dealt with in the various branches of physics. In general, modern physics, especially through the energetic reaction (Duhem, Ostwald) against the mechanism of classical physics and through the theories of relativity and quanta (v.), recognizes the irreducibility of q. to quantity alone and local motion, as well as the necessity of qualitative elements for explaining physical reality: such energy (v.), especially electromagnetic and gravitational energy. The merit of mechanism is that it brought to light the necessity, fundamental in all modern physics, of subjecting q. as well to a rigorous process of measurement and mathematical calculation. The quantitative method, however, is not opposed to the affirmation of sensible q. and, even according to the principles of scholastic philosophy, may be introduced into the study of q., insofar as these too have a quantity, the « intensio » or « quantitas virtutis », measurable by the quantitative effects it produces: « nam quantitas virtutis secundum effectus mensurari solit » (St. Thomas, Comp. theol., chap. 216); thus, the intensity or degree of a body’s temperature is measured by the expansion of mercury in a thermometer. With regard to the subjective or psychological element corresponding to sensible q. and its measurement, the fruitful investigations of modern psychology are well known, particularly those of Fechner, Weber, Paulsen, and Wundt (v. SENSATION).
The problem of the intensity and increase of q. received particular consideration in scholasticism because of its theological implications in the question of the increase of Grace, charity, and supernatural virtues. Essentially, two principal explanations were proposed: the first, defended by Scotus (Commentaria ad I librum Magistri sent., dist. XVII, qq. 4, 5, 6; I, Quaracchi 1912, p. 834 ff.) and by Suárez (Disp. metaph., disp. 46, ed. Vivès, XXVI, Paris 1861, p. 753 ff.), explains the increase of q. by the addition (additio) of new entities, just as weight can be increased by adding new weights; the other, upheld by St. Thomas (Sum. Theol., 1a-2ae, q. 52, a. 1 and 2; 2a-2ae, q. 24, a. 4-7; De virt., q. 1, a. 11), explains the increase « per hoc quod subiectum magis vel minus perfecte participat unam et eandem formam », that is, insofar as one and the same form actualizes the potential capacity of the subject ever more perfectly, under the action of an external efficient cause or through the repetition of acts. In reality, the Scotist-Suárezian opinion explains only the increase of the « quantitas per accidens » of q. (e.g., to have a greater quantity of heat, it is enough to add new bodies having the same degree of temperature), whereas to explain the increase of intensity or « quantitas intensiva » (e.g., the increase of temperature), it is necessary to resort to the second explanation.