It. Qualità percettiva; Fr. Qualité perceptuelle; Germ. Wahrnehmungsqualität; Span. Cualidad perceptiva. The expression “perceptual quality” comes from the Latin words perceptiva (feminine form of perceptivus, Medieval adjective based on percipio, “to receive” or “to perceive”) and qualitas (derived from qualis and modeled on the Greek ποιότης, poiótēs, “quality”, and the corresponding ποῖος, poîos, “which”, “what”). Since “quality” is often used, in a perceptual context, as a synonym of “property”, it may be said that a “perceptual quality” generally indicates a distinctive property ascribed to a phenomenon perceived by our senses, such as a region of one’s visual field (square or red), a smell (fruity), a flavor (sweet), a sound (loud), or a tactile sensation (soft).
From a historical point of view, the discussion on perceptual qualities starts from Democritus and his distinction between primary and secondary qualities, the former belonging to the object itself, the latter to the individual subject. This distinction was revived in the 17th century during the age of the scientific revolution, starting with Galilei, and continued in the subsequent century. Since the end of the 19th century, the discussion on perception and its qualities has become a special object of interest, thanks to the development of new perspectives, among which it is important to mention phenomenological, Gestaltist, and analytic ones.
Contemporary authors wonder whether perceptual qualities shall be ascribed to the object or the subject of perception, thus resuming the issue of primary and secondary properties. Moreover, the presence of an underlying object may be questioned and the subject may be conceived differently, in an individual or intersubjective sense, as distinguished by transcendental conditions or linguistic acts. The main positions involved in this debate are direct and indirect realism, adverbial theory, phenomenology, and Object-Oriented Ontology (OOO).
Realism sees perception as a form of sensible knowledge of objects existing outside the human mind. Direct (naive) realism considers perception as a non-mediated process, where an object is directly grasped through human senses, whereas indirect realism implies the idea of a mediation, which makes perception a representative process. In the former case, perception consists of a relation, in the latter it is a form of representation.
The naive realist and Gestaltist Paolo Bozzi states that there is no difference between primary and secondary qualities, since they are both perceivable and belong to the object of perception (Bozzi 1990). Another naive realist, John Campbell, thinks that, for instance, the properties of shape are categorical properties of the object (Campbell 1996). In both cases, perceptual qualities belong to a mind-independent object and are directly seized by the human mind. Indirect realism (representationalism) gives particular importance to perceptual qualities: perception is a mediate consciousness of a set of qualities (or qualia) constituting an object, not of a whole object with its qualities. Perceptual properties do not intrinsically belong to an object, but constitute a particular experience of it (Tye 2002): for example, the experience of smelling consists in predicating olfactory qualities (Batty 2010), not in grasping an olfactory object. The use of “predicating” means that phenomenal experience is similar to cognition, even if they do not coincide: phenomenal predication involves the presentation of something, which may be absent in cognition (Pautz 2020).
In both direct and indirect realism there are reductionist and anti-reductionist views. According to Gestalt psychology, the relation between the perceived qualities, coming from a stimulus array, and the neural structures of human mind, is isomorphic: the array corresponds to the underlying objective brain processes, allowing an analysis of the perceived thing (Köhler 1920). Reductive-externalist representationalists are not interested in grasping the whole object, but in the reduction of sensible properties to physical properties: a certain brain state occurs only if the property is present before the subject (Dretske 1995). For what concerns non-reductionist positions, Bozzi thinks that the presentation of certain qualities does not necessarily correspond to precise neural processes, since the science of perception proceeds iuxta propria principia, thus challenging neuroscientific approaches (Bozzi 1990). On the representationalist side, experience is more than a simple set of neural states, so that sensible qualities cannot be identified with neural qualities (Pautz 2020).
Adverbial theory is phenomenologically rooted in Brentano’s thought (Chisholm 1957), though, just as representationalism, conceives perception as an elaboration of qualities, not of whole objects. However, perceptual process is not representative, but semantic. More precisely, adverbialism holds that perception consists of an agent involved in an event of a certain kind, an individual experience which is modified and expressed through adverbs (D’Ambrosio 2019): for instance, I do not “see a red sphere”, but “see redly and spherely”. In the context of this theory, there are no objects involved, so qualities are the aim of perception. Many objections were addressed in the 1970s to adverbialism, such as the many-property and many-relation problems (Jackson 1975), but a more consistent response has recently been given. According to D’ambrosio (2019), adverbialism must get closer to relational theory and consider adverbial perception as aiming to relational perception, even if it is not always successful. If I say that “I see redly and spherely” and the red thing perceived is actually a sphere, this is an event of relational perception. If, on the contrary, I get closer to the red thing and realize that it is a cylinder, perception is non-relational. The role of the object seems here revalued, although perceptual qualities are still the hard core of this theory.
Phenomenology may be declined in both realist and idealist ways. In its classical version, it sustains transcendental idealism. The existence of external reality is neither asserted, nor denied, but what is experienced as an outer object is considered only in relation to the I and its transcendental structures. Unlike representationalism and adverbialism, according to phenomenology the object as a whole is perceived, as the aim of an intentional act of consciousness, even if it is grounded in passive synthesis (Husserl 2001; Merleau-Ponty 2012). From this point of view, qualities are the changing features of the perceived objects, which are in an intentional relation with the I. Albeit these changes, some constant qualities may be individuated through the method of eidetic variation (Husserl 1975). For instance, the dimensions or the colour of a sphere cannot define a sphere as such, but its consistent curvature and roundness do, belonging to its eidos.
Experimental phenomenology derives from Gestalt theory and has a realist background (Epstein, Hatfield 1994), which may be declined in both direct (Bozzi 1990) and indirect versions (Arnheim 1974). According to this approach, perception takes place through configurations (Gestalten) of our mind and the subject becomes aware of them only when it encounters the external object, which tends to be persistent, as Wertheimer (2012) shows in his demonstration of the phi-phenomenon. Perceptual qualities are Gestaltqualitäten, so perception is a field process, where qualities are part of a comprehensive whole and depend on it (von Ehrenfels 1937). Classical and experimental approaches, albeit their differences, find a common ground in considering perceptual qualities as included in a transcendental structure, which belongs to human mind and is discovered through experience. This applies also to tertiary qualities, which are not purely perceptual: though referring to a perceptual aspect (color, sound, etc.), they involve also emotional and/or cognitive aspects. Experimental phenomenology, thanks to the ecological theory (Gibson 1979) and its concept of affordance, conceives of perceptual qualities as intersubjective and expressive, leading to an extended idea of perception.
OOO has developed an original concept of perceptual qualities, based on indirect realism and on the refusal of correlationism. The theory of the quadruple object (Harman 2011) denies any correlation between subject and object: reality is made only of existing objects, which are autonomous, and relations between them are limited to their sensual dimension; moreover, objects are not mere bundles of qualities, but independent wholes whose qualities are in tension with them. Specifically, this leads to a separation between a real object (RO) and a sensual one (SO); objects are also seen in relation to their corresponding qualities, which are real (RQ) and sensual (SQ). Since perception is the tension between SO and SQ, the latter are identified as perceptual qualities. Harman also interprets Husserl, saying that perceptual qualities are changing and depending from our perspective: they give access only to the phenomenal aspects of an object. RO and its RQ are not knowable by us, since they are completely independent and withdrawn from other objects. Harman is also inspired by Husserl for what concerns the definition of eidetic knowledge, resulting from the tension between SO and RQ, where the latter are the eidetic and unchanging qualities of the object. Shortly, perceptual qualities (SQ) are changing and inessential, being phenomenal versions of essential qualities (RQ).
The abovementioned theories on perceptual qualities also try to explain how properties shall be conceived in illusory or hallucinatory phenomena.
According to direct realism and phenomenology, illusions are ways in which reality presents itself. In the Müller-Lyer illusion, even if the two segments, scientifically speaking, have the same length, perceptually speaking they are different, because they appear to us as different (Taddio 2020). In this context, illusion is defined as deceptive only because we commit the “stimulus error” (Köhler 1929), confusing phenomenal experience with scientific knowledge. The quality that we see in the so-called “illusory phenomenon” is not false, but a perceptual property as occurs in ordinary perception, belonging to the object as such. For what concerns hallucinations, naive realists think that qualities appear to us differently than in perceptual phenomena: hallucinatory experience is of a completely different kind (Martin 2004). In indirect realism, illusory and hallucinatory experiences are considered equivalent and do not differ from regular perceptual experience; therefore, perceptual qualities are not apparently different from qualities grasped in illusory or hallucinatory phenomena.
In the most recent forms of adverbialism (D’Ambrosio 2019), illusion is understood as a form of relational perception, which does not satisfy the success conditions of adverbial perception (I hear a growl and relate it to a bear, but it is just a friend of mine), whereas hallucination is a form of experience where the subject fails to perceive relationally at all (I hear a growl, but there is nobody growling). Perceptual qualities (growly) are then related to the wrong object (illusion) or to a non-veridical one (hallucination).
OOO has not particularly focused on illusions and hallucinations. It generally sees illusions as a result of the rift between real and sensual objects: every sensual object is illusory as such and the same can be said for qualities (Morton 2013). For what concerns hallucinations, Harman does not mention any particular difference with illusions (Harman 2007), whereas Morton sees hallucinatory objects and qualities as different than real ones (Morton 2011).
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