PERCEPTUALISM. — In philosophy, it is that theory of sensory knowledge which affirms the ordinary formal objectivity of sensible qualities, whether primary (q.v.) or secondary. It is distinguished from the interpretation that affirms the formal objectivity of primary qualities and the causal objectivity of secondary qualities, ultimately resolving the secondary formalities into the proper mode of external sensation.
The question was already open in the time of the pre-Socratics, and it was explicitly posed by the atomists: “Convention is colour, convention is sweet, convention is bitter; in reality only atoms or the void” (Democritus, fr. 123). Later, the Sophists (q.v.) made a sharper critique of sensation, and from the denial of the objectivity of all sensible qualities they moved to the affirmation of integral relativism. To a similar conclusion, after Berkeleyan and Humean empiricism, modern phenomenists also arrived.
The denial of the ancient atomists, which according to Aristotle was inspired by the Pythagorean ideal of reducing all reality to numbers (De caelo, III, ch. 4, 303, 9) and which constitutes the central nucleus of mechanism (q.v.), was taken up again by Galileo (cf. Il Saggiatore, nn. 48 ff.): “Descartes (cf. Meditations, VI, ed. Adam-Tannery, VII, p. 50). Subsequently, Locke distinguished the primary qualities, belonging to the real objects of perception (solidity, extension, figure, motion or rest, number), from secondary qualities (colour, sound, taste, etc.); the perception of these latter, according to Locke, is objective only in a causal sense, inasmuch as in existing reality the capacities to produce in us the sensation of the secondary quality through the primary ones (Essay Concerning Human Understanding, I, ch. 8). Modern scientists generally incline towards this view, because present-day scientific experiments, especially in the so-called errors of the senses, find a more ready explanation in the interpretative hypothesis.
In Aristotelian and medieval philosophy, which did not escape the obvious observation of the errors of the senses, the perceptualist conception prevailed (e.g., St Thomas Aquinas, cf. Summa Theologiae, I, q. 17, a. 2; q. 85, a. 6). In contemporary Neoscholasticism, the question presents itself under a threefold aspect: critical, concerning the value of sensory knowledge; bio-psychological, concerning the process and nature of sensation (q.v.) and perception (q.v.); and physical-cosmological, concerning the nature of quality in itself (q.V. Qualità). From the critical standpoint, the problem arises from the distinction between the “sensible per se” into sensible qualities “proper” to a single sense (colour, sound, taste, etc.) and sensible qualities “common” to several senses (motion, rest, number, figure, size). With regard to common sensibles, Neoscholastic thought is in agreement in admitting their ordinary formal objectivity; concerning proper sensibles, the purely mechanistic explanation is mostly excluded, and it is admitted that to sensations there correspond proportionate active qualities that influence the sense. By some (Mattius, Mercier, Donat, Frobes, Gründer, etc.), however, the strict objectivity of proper sensibles is denied, understood in their formal sense; these authors base themselves on the specific unintelligibility of proper sensibles and on the easier explanation that the interpretative view allows of experience. Others (Liberatore, Pesch, Farges, Gredt, Geny, Vanni-Rovighi, etc.) affirm their ordinary formal objectivity, observing that proper sensibles are given in knowledge (which is assimilation and not interpretation) as sensibles per se to a greater degree than common sensibles, inasmuch as it is through the “proper” that the “common” is apprehended. At the present state of research, it must be acknowledged that neither of the two opinions is able to provide a definitive answer to the individual difficulties that recur under both the scientific and the philosophical aspects.