PERCEPTIONISM. - In philosophy, it is that theory of sensible knowledge which affirms the ordinary formal objectivity of sensible qualities, both primary (v.) and secondary. It is distinguished from interpretationism, which affirms the formal objectivity of primary qualities and the causal objectivity of secondary ones, ultimately resolving secondary formality into the proper mode of exterior sensation.
The question had been open since the time of the Presocratics, and it was posed explicitly by the atomists: "By convention color, by convention sweet, by convention bitter; in reality only atoms and void" (Democritus, frg. 125). Later the Sophists (v.) made a more acute critique of sensation, and from the denial of the objectivity of all sensible qualities they passed to the affirmation of integral relativism. Modern phenomenalists reached an analogous conclusion after the empiricism of Berkeley and Hume. The denial of the ancient atomists, which according to Aristotle was inspired by the Pythagorean ideal of reducing all reality to numbers (De coelo, III, ch. 4, 303, 9) and which constitutes the central nucleus of mechanism (v.), was taken up again by Galileo (cf. Saggiatore, n. 48 ff.) and Descartes (cf. Medit., VI, ed. Adam-Tannery, VII, p. 80). Later Locke distinguished primary qualities, belonging to the real objects of perception (solidity, extension, figure, motion or rest, number), from secondary qualities (color, sound, taste, etc.); the perception of the latter, according to Locke, is objective only in the causal sense, insofar as in reality there exist the capacities to produce in us the sensation of the secondary quality by means of the primary ones (Saggio sull'umano intelletto, I, ch. 8). Modern scientists generally orient themselves in this direction, because present-day scientific experiments, especially in the so-called errors of sense, find easier explanation in the interpretationist hypothesis.
In Aristotelian and medieval philosophy, which did not overlook the obvious observation of the error of the senses, the percezionistic conception prevails (e.g., St. Thomas, cf. Sum. Theol., 1ª, q. 17, a. 2; q. 85, a. 6). In contemporary Neoscholasticism the question presents itself under a threefold aspect: critical, concerning the value of sensible knowledge; bio-psychological, concerning the process and nature of sensation (v.) and perception (v.); and physical-cosmological, concerning the nature of quality in itself (v. QUALITÀ). Under the critical aspect the problem arises starting from the distinction of the "sensible per se" into sensible "proper" to a single sense
(color, sound, taste, etc.) and sensible "common" to several senses (motion, rest, number, figure, magnitude). As regards the common sensible, Neoscholastic thought is in agreement in admitting its ordinary formal objectivity; with respect to the proper sensible, the purely mechanistic explanation is for the most part excluded, admitting that to sensations there correspond proportionate active qualities that act upon the sense. By some (Mattiussi, Mercier, Donat, Frobes, Gründer, etc.), on the other hand, the objectivity in the strict sense of the proper sensible, understood in its formal sense, is denied; such authors base themselves on the specific non-intelligibility of the proper sensible and on the easier explanation that interpretationism allows of experiences. Others (Liberatore, Pesch, Farges, Gredt, Geny, Vanni-Rovighi, etc.) affirm ordinary formal objectivity, observing that the proper sensible is given in knowledge (which is assimilation and not interpretation) as sensible per se with greater title than the common, insofar as it is through the "proper" that the "common" is apprehended. In the present state of research it is objective to hold that neither of the two opinions is capable of giving a definitive answer to the individual difficulties that reappear under both the scientific and the philosophical aspect.
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