Aristotle: Perception

Mark Eli Kalderon presents an original study of perception, taking as its starting point a puzzle in Empedocles' theory of vision: if perception is a mode of material assimilation, how can we perceive colors at a distance? Kalderon argues that the theory of perception offered by Aristotle in answer to the puzzle is both attractive and defensible.

I argue that a study of the Nicomachean Ethics and of the Parva Naturalia shows that Aristotle had a notion of attention. This notion captures the common aspects of apparently different phenomena like perceiving something vividly, being distracted by a loud sound or by a musical piece, focusing on a geometrical problem. For Aristotle, these phenomena involve a specific selectivity that is the outcome of the competition between different cognitive stimuli. This selectivity is attention. I argue that Aristotle studied the (...) common aspects of the physiological processes at the basis of attention and its connection with pleasure. His notion can explain perceptual attention and intellectual attention as voluntary or involuntary phenomena. In addition, it sheds light on how attention and enjoyment can enhance our cognitive activities. (shrink)

A puzzle about the presentation of objects located at a distance is seen to animate Aristotle's account of transparency in De Anima and De Sensu.

A reply to Todd Ganson’s “Was Aristotle a Naïve Realist”, a talk for a conference in Gothenburg Sweden 12-14 June 2015 entitled The Mechanisms of Sense Perception in Aristotle and the Aristotelian Tradition.

Aristotle’s works on natural science show that he was aware of the affective powers of colour. At De an. 421a13, for example, he writes that hard-eyed animals can only discriminate between frightening and non-frightening colours. In the Nicomachean Ethics, furthermore, colours are the source of pleasures and delight. These pleasures, unlike the pleasures of touch and taste, neither corrupt us nor make us wiser. Aristotle’s views on the affective powers of colours raise a question about the limits he seems to (...) place on the affective powers of pictures at De an. 427b15-24, where he implies that pictures do not affect us immediately. In this paper, I examine the contrast between the affective powers of colour and the affective powers of pictures. I argue that colours can give rise to pleasure and pain in themselves and generate emotions incidentally. Similarly, pictures can please us or affect us in themselves and incidentally. In light of this account, I suggest that, on a plausible reading of De an. 427b15-24, the affective powers of pictures as mimetic objects are not immediate because they require an intervening cause in order to be effective. The representations of pictures and statues affect us either with the mediation of deception or with the mediation of interpretation. (shrink)

Alexander of Aphrodisias picks up Aristotle’s insufficient treatment of simultaneous perception and develops an adequate solution for the problem, thereby offering an account of the unity of perceptual consciousness—the single mental activity of a single subject with complex content. I show the adequacy of the solution by using as criteria the requirements that have been identified by Aristotle and approved (and explained) by Alexander. I analyze Alexander’s solution in two turns. First, with respect to heterogeneous perceptibles, Alexander adopts and reformulates (...) Aristotle’s metaphorical account invoking the analogy with a point. Second, with respect to homogeneous opposites, accordingly, perception is judgement, but it involves physical changes in diverse parts of the primary sense-organ. By this account Alexander resolves the issue of the unity of the subject on the level of the capacity of the soul, and coordinates the complexity of content with the complexity on the physical level. In addition to being adequate, the solution is faithful to Aristotle. I suggest that the interpretative decisions Alexander makes (the clarification of the analogy; the reference he finds to the analogy; the two components of the solution, judgement and parts of the organ) form an ingenious extension of Aristotle’s treatment. Interestingly, even though many elements in Alexander’s interpretation are taken up by modern commentators, no one has followed it in its entirety, nor even treated it in its own right. (shrink)

In this ambitious first book, Erick Raphael Jiménez argues that a good model for understanding Aristotle’s concept of mind (nous) lies in Aristotle’s account of the perception of time. This “time-perception model” of mind and its activity, thinking, bridges a gap between Jiménez’s unorthodox readings of Aristotelian mind and its objects. The book will attract the interest of specialists in Aristotle’s psychology, as well as other scholars interested in Aristotle’s concept of mind and its influence, for instance, theologians interested in (...) Aristotle’s tantalizing discussion of active or maker mind in De Anima 3.5 (DA). (shrink)

For Aristotle, human cognition has a lot in common both with non-human animal cognition and with divine cognition. With non-human animals, humans share a non-rational part of the soul and non-rational cognitive faculties (DA 427b6–14, NE 1102b29 and EE 1219b24–6). With gods, humans share a rational part of the soul and rational cognitive faculties (NE 1177b17– 1178a8). The rational part and the non-rational part of the soul, however, coexist and cooperate only in human souls (NE 1102b26–9, EE 1219b28–31). In this (...) chapter, I show that a study of this cooperation helps to uncover some distinctive aspects of human cognition and desire. (shrink)

Aristotle held that perception consists in the reception of external sensory qualities (or sensible forms) in the sensorium. This idea is repeated in many forms in contemporary philosophy, including, with regard to vision, in the idea (still not firmly rejected) that the retinal image consists of points of colour. In fact, this is false. Colour is a quality that is constructed by the visual system, and though it is possible to be a realist about colour, it is completely misleading to (...) think of it as received by the retina. Moreover, such supposedly “charitable” interpretations of Aristotle’s doctrines, based on miscon- ceptions of perception-science, distort our understanding of his historical context. (shrink)

This is an excellent translation of Aristotle's De Anima or On the Soul, part of C.D.C. Reeve's impressive ongoing project of translating Aristotle's works for the New Hackett Aristotle. Reeve's translation is careful and accurate, committed to faithfully rendering Aristotle into English while making him as readable as possible. This edition features excellent notes that will greatly assist readers (especially in their inclusion of related passages that illuminate the sections they annotate) and an introduction that situates the work within Aristotle's (...) scientific method and his overall view of reality. (shrink)

In De Anima I 4, Aristotle describes the intellect (nous) as a sort of substance, separate and incorruptible. Myles Burnyeat and Lloyd Gerson take this as proof that, for Aristotle, the intellect is a separate eternal entity, not a power belonging to individual humans. Against this reading, I show that this passage does not express Aristotle’s own views, but dialectically examines a reputable position (endoxon) about the intellect that seems to show that it can be subject to change. The passage’s (...) argument for the indestructibility of intellect via an analogy to perception does not fit with Aristotle’s own views. Aristotle thinks that perception operates via bodily organs, but denies this of understanding. He also requires separability from the body for indestructibility, something this analogy rules out. However, Aristotle’s Platonist interlocutors may well endorse such an argument. My dialectical interpretation best resolves the interpretative difficulties and explains its place in the larger context, Aristotle’s discussion of Platonist views on the soul. Aristotle presents a challenge to his insistence that the soul is subject to change, dialectically resolves that challenge, and then ends by reserving the right to give a different account of the intellect. (shrink)

As we now know, most, if not all, philosophers in the High Middle Ages agreed that what we immediately perceive are external objects and that the immediate object of perception must not be some image present to the mind. Yet most — but not all — philosophers in the High Middle Ages also held, following Aristotle, that perception is a process wherein the percipient takes on the likeness of the external object. This likeness — called a species — is a (...) representation (of some sort) by means of which we immediately perceive external objects. But how can perception be at once direct — or immediate — and at the same time by way of representations? The usual answer here was that the species represents the external object to some percipient even though the species itself is not at all perceived: the species is that by which I perceive and not that which I perceive. John Buridan defends this traditional view — call it direct realism with representations. However, just a couple of decades before Buridan, one of the more important philosophers at Paris, Durand of St.-Pourçain, had already rejected direct realism with representation. Durand defends what I will call direct realism without representations. On his view, a species is not at all necessary during overtly direct forms of perception, neither as cause nor as representation. This paper has two parts. In the first part, I will discuss some of the more interesting arguments that Durand makes against direct realism with representations. In the second part, I will look at Buridan's defense of the view. -/- . (shrink)

Sense experience, naïvely conceived, is a way of knowing perceptible properties: the colors, sounds, smells, flavors, and textures in our perceptual environment. So conceived, ordinary experience presents the perceiver with the essential nature of a property like Sky Blue or Middle C, such that how the property appears in experience is identical to how it essentially is. In antiquity, as today, it was controversial whether sense experience could meet the conditions for knowledge implicit in this naïve conception. Aristotle was a (...) partisan in this debate, but his position is poorly understood. This dissertation examines how Aristotle’s perceptual psychology responds to ancient challenges to the naïve conception, and so articulates his defense of perceptual knowledge. -/- Aristotle’s defense relies on an ontology of “perceptual qualities"—a core class of perceptible properties—according to which those qualities, despite having a perceiver-independent essence rooted in the physics of causation and affection, nevertheless can be present in experience as they essentially are. Chapter 1 defends this realist and objectivist reading against competing interpretations, which overlook a crucial distinction between perceptual qualities and perceptual objects. Chapter 2 presents Aristotle’s ontology as a physicalism that uncharacteristically allows for perceptual qualities to appear in experience as they essentially are. -/- This ontology informs Aristotle’s account of the psychological conditions under which perceivers actually are presented with the essence of perceptual qualities. The locus of this account is an obscure passage where Aristotle purports to show that the senses “discriminate” perceptual qualities because the senses are “mean states” (An. 2.11, 424a5–7). Chapter 3 develops a comprehensive interpretation of the sensory mean state, which Chapter 4 uses to elucidate Aristotle’s argument for sensory discrimination. Sensory discrimination turns out to be a process in which the essence of a perceptual quality comes to be present in the affection it produces in a perceiver. For Aristotle, this shows that sense perception meets a condition for knowledge that his predecessors, including Plato, thought it could not meet. For in his view, but not in theirs, sense experience shares in both the truth and the essence of the qualities it perceives. (shrink)

I examine the passages where Aristotle maintains that intellectual activity employs φαντάσματα (images) and argue that he requires awareness of the relevant images. This, together with Aristotle’s claims about the universality of understanding, gives us reason to reject the interpretation of Michael Wedin and Victor Caston, on which φαντάσματα serve as the material basis for thinking. I develop a new interpretation by unpacking the comparison Aristotle makes to the role of diagrams in doing geometry. In theoretical understanding of mathematical and (...) natural beings, we usually need to employ appropriate φαντάσματα in order to grasp explanatory connections. Aristotle does not, however, commit himself to thinking that images are required for exercising all theoretical understanding. Understanding immaterial things, in particular, may not involve employing phantasmata. Thus the connection that Aristotle makes between images and understanding does not rule out the possibility that human intellectual activity could occur apart from the body. (shrink)

Is life a simple result of a conjunction of physico-chemical processes? Can be reduced to a mere juxtaposition of spatially determined events? What epistemology or world-view allows us to comprehend it? Aristotle built a novel philosophical system in which nature is a dynamical totality which is in constant movement. Life is a manifestation of it, and is formed and governed by the psyche. Psyche is the organizational principle of the different biological levels: nutritive, perceptive and intelective. Driesch's crucial experiment provided (...) empirical proof of the principle of life, which he called entelechy. Entelechy is an intensive manifoldness and cannot be comprehended by the usual extensive parameters. The entelechian's own ambiance is duration. This allows the reintroduction of the concept of teleology in the sphere of the living, understood not as a final cause, but as an order born from desire and leading to action. (shrink)

Aristotle sometimes claims that the perception of special perceptibles by their proper sense is unerring. This claim is striking, since it might seem that we quite often misperceive things like colours, sounds and smells. Aristotle also claims that the perception of common perceptibles is more prone to error than the perception of special perceptibles. This is puzzling in its own right, and also places constraints on the interpretation of. I argue that reading Alexander of Aphrodisias on perceptual error can help (...) to make good sense of both of Aristotle’s claims. (shrink)

How many senses do humans possess? Five external senses, as most cultures have it—sight, hearing, touch, smell, and taste? Should proprioception, kinaesthesia, thirst, and pain be included, under the rubric bodily sense? What about the perception of time and the sense of number? Such questions reduce to two. 1. How do we distinguish a sense from other sorts of information-receiving faculties? 2. By what principle do we distinguish the senses? Aristotle discussed these questions in the De Anima. H. P. Grice (...) revived them in 1967. More recently, they have taken on fresh interest as a result of a collection of essays edited by Fiona Macpherson. This entry reviews some approaches to these questions and advances some new ideas for the reader’s consideration. It proposes that the senses constitute an integrated learning system, membership in which answers question 1. It also proposes that the modalities can be distinguished from one another in two ways, by the means of information pick-up and by the kinds of activity that a perceiver undertakes to make use of them. (shrink)

I reconstruct Aristotle’s reasons for thinking that the intellect cannot have a bodily organ. I present Aristotle’s account of the aboutness or intentionality of cognitive states, both perceptual and intellectual. On my interpretation, Aristotle’s account is based around the notion of cognitive powers taking on forms in a special preservative way. Based on this account, Aristotle argues that no physical structure could enable a bodily part or combination of bodily parts to produce or determine the full range of forms that (...) the human intellect can understand. For Aristotle, cognitive powers with bodily organs are always spatiotemporally limited, but the understanding is not. Aristotle claims that our understanding applies to all instances of the thing understood wherever and whenever they exist. On Aristotle’s own account the intellect in its nature is only “potential,” it does not actually possess any form. Thus nothing prevents it from possessing all forms. (shrink)

Kazimir Malevich’s style of Suprematist painting represents the inauguration of nothing less than a new form of culture premised upon a demolition of the Western tradition’s reifying habits of objective thought. In ridding his canvases of all objects and mimetic conventions, Malevich sought to reconfigure human perception in such a way as to open consciousness to alternative modes of organization and signification. In this paper, I argue that Malevich’s revolutionary aesthetic strategy can be illuminated by a return to the very (...) basis of this tradition, namely by a reconsideration of Aristotle’s account in De Anima III.2 of the initial possibility of objective perception as such. (shrink)

Most philosophers in the High Middle Ages agreed that what we immediately perceive are external objects. Yet most philosophers in the High Middle Ages also held, following Aristotle, that perception is a process wherein the perceiver takes on the form or likeness of the external object. This form or likeness — called a species — is a representation by means of which we immediately perceive the external object. Thomas Aquinas defended this thesis in one form, and Durand of St.-Pourçain, his (...) Dominican successor, rejects it. This paper explores Durand's novel criticism of Aquinas's species-theory of cognition. I first develop and defend a new interpretation of Durand's central criticism of Aquinas's theory of cognition. I close with some considerations about Durand's alternative to the theory. -/- . (shrink)

In this paper I consider two related issues raised by Aristotle 's treatment of hearing and sounds. The first concerns the kinds of changes Aristotle takes to occur, in both perceptual medium and sense organs, when a perceiver hears a sounding object. The second issue concerns Aristotle 's views on the nature and location of the proper objects of auditory perception. I argue that Aristotle 's views on these topics are not what they have sometimes been taken to be, and (...) that when rightly understood they compare favourably in many respects with leading contemporary accounts. (shrink)

Aristotle’s treatment of tactility is at odds with the hierarchical order of psyche’s faculties. Touching is the commonest and lowest power; it is possessed by all sentient beings; thinking is, on the contrary, the highest faculty that distinguishes human beings. Yet, while Aristotle maintains against some of his predecessors that to think is not to sense, he nevertheless posits a causal link between practical intelligence and tactility and even describes noetic activity as a certain kind of touch. This essay elucidates (...) Aristotle’s analysis of the sense of touch in De anima and argues that tactility provides a paradigm for sensitivity in general and in particular for the reflexivity of sensation whereby the senses disclose not only what they are sensing but also that they are sensing. This feature, it is argued, has epistemological and ontological consequences. The sense of touch testifies to the physical presence of material beings and provides an empirical verification of substance’s essential feature, namely, self-reference. (shrink)

In disagreement with Claudia Baracchi’s controversial thesis that there is a “simultaneity and indissolubility” if not an “identity” of intelligence (nous) and perception (aisthēsis) at the core of Aristotle’s philosophy, I will argue that Aristotle maintains a fundamental distinction between these cognitive faculties. My goal in this paper is to examine specific parts of two central and complex passages, VI.8, 1142a12-30 and VI.11, 1143a33-b15, from the Nicomachean Ethics to show that Baracchi’s view is unpersuasive. I will show that Aristotle considers (...) nous to be a different capacity than mere perception and one through which particulars and indemonstrable principles become intelligible. Moreover, I will show that Aristotle considers that the objects of nous differ in kind from those that sensation (our senses) and perception (inference from our senses) grasp. After examining critically Baracchi’s thesis in light of a close reading of those two relevant passages, I will conclude the paper by showing the significance of Aristotle’s claim that a state is defined in terms of its objects of apprehension for understanding the distinction between nous and aisthēsis. (shrink)

The article focuses on an unexamined so far aspect of byzantine philosophy, namely the influence of Arabic philosophy upon byzantine thinkers. Despite the vicinity of Byzantium and Arabic territories, the philosophical interactions were minimal. Scholarios claimed, in a dedicatory epistle to Constantine Paleologus (1405-1453), that he had studied the treatises of Avicenna, Averroes, and other Arab and Persian philosophers. He admitted that Averroes was beyond doubt the best commentator of Aristotle. Scholarios acknowledged that the study of the Arabs contributed immensely (...) to his philosophical education and particularly to the proper understanding of the Aristotelian philosophy (Scholarios, vii.1-6). Scholarios aimed at the enrichment and renewal of Byzantine philosophy. Besides his high esteem for Arabic philosophy, he devoted a large part of his life translating and paraphrasing the Scholastics, especially Aquinas. Despite Scolarios’ claim about his erudition on the Arab and Persian philosophers, a detailed examination of his works proves that in most cases he simply reproduced and incorporated sections from Aquinas’ works, without resorting to the original sources. Scholarios’ references to the Arabs are multiplied in his translations and compendia of Aquinas’ works, but are reduced significantly in his original texts. Frequently Scholarios refrained from mentioning the Arab philosophers, despite the fact that he commented on Aquinas’ passages, which are dedicated to Avicenna or Averroes. On the contrary, Scholarios did not avoid mentioning in detail Aquinas’ Christian or ancient Greek sources. It is obvious that Scholarios did not have a consistent approach towards Arabic philosophy. Most of the times he reproduced loosely Aquinas’ passages, where the latter commented on the Arabs. (shrink)

At De Anima II 5, 417b17, Aristotle says, ‘The first transition (πρώτη μεταβολή) in that which can perceive is brought about by the parent, and when it is born it already has [the faculty of] sense-perception in the same way as it has knowledge. Actual sense-perception is so spoken of in the same way as contemplation.’ The purpose of this paper is to determine the nature of first transitions.

This paper offers a new interpretation of Aristotle’s identification, in De Anima 2.5, of αἴσθησις with an ἀλλοίωσίς τις that is not ‘a kind of destruction of something by its contrary’. Drawing on a passage from Metaphysics Iota 5, it argues that when so described, what is referred to as an ἀλλοίωσίς τις is not a uniquely perceptual alteration.

The sense of smell occupies a peculiar intermediate position within Aristotle's theory of sense perception: odours, like colours and sounds, are perceived at a distance through an external medium of air or water; yet in their nature they are intimately related to flavours, the proper objects of taste, which for Aristotle is a form of touch. In this paper, I examine Aristotle's claims about odour and smell, especially in De Anima II.9 and De Sensu 5, to see what light they (...) shed on his theory of sense perception more generally. In the first half, I argue that neither of the two most influential recent ways of understanding Aristotle's theory of perception can adequately account for what he says about the sense of smell. In the second half, I offer my own positive account, considering and resolving various puzzles raised by Aristotle's claims about the nature of odour and its relation to flavour. Finally, I conclude that Aristotle's discussions of odour and smell suggest a plausible and interesting way of understanding the relationship, on his view, between ordinary, material changes in the sense organs and the activation of the capacity to perceive, considered merely as such. (shrink)

In De Anima II 5, 417a21-b16, Aristotle makes a number of distinctions between types of transitions, affections, and alterations. The objective of this paper is to sort out the relationships between these distinctions by means of determining which of the distinguished types of change can be coextensive and which cannot, and which can overlap and which cannot. From the results of this analysis, an interpretation of 417a21-b16 is then constructed that differs from previous interpretations in certain important respects, chief among (...) which is its characterization of transitions from first potentiality to first actuality, e.g., learning, not as `ordinary', but rather as acquisitions of natural dispositions or faculties. (shrink)

Arguably one of the most fundamental phase shifts that occurred in the intellectual history of Western culture involved the ontological reduction of secondary qualities to primary qualities. To say the least, this reduction worked to undermine the foundations undergirding Aristotelian thought in support of a scientific view of the world based strictly on an examination of the real—primary— qualities of things. In this essay, I identify the so-called “Causal Argument” for a reductive view of secondary qualities and seek to deflect (...) this challenge by deriving some plausible consequences that support a non-reductive view of secondary qualities from an Aristotelian view (via the philosophical commentary of Thomas Aquinas). Specifically, my argument has two facets. First, I show that Aristotle’s view both implies recognition of the extramental existence of secondary qualities and is a prima facia natural view to take regarding the ontology of secondary qualities. Second, I show that the Causal Argument, which is thought to undermine a natural view of secondary qualities as real things, loses its bite when one examines perception in light the ontological relationship among the categories of quality, quantity and substance. (shrink)

In the first part of the paper, I’ll rehearse an argument that perceiving that we see and hear isn’t a special case of perception in Aristotle but is rather a necessary condition for any perception whatsoever: the turning of one’s attention to the affection of the sense organs. In the second part of the paper, I’ll consider the thesis that the activity of the active intellect is analogous to perceiving that we see and hear.

I argue that the idea that mental states possess a primitive intentionality in virtue of which they are able to represent or ‘intend’ putative particulars derives largely from Brentano‘s misinterpretation of Aristotle and the scholastics, and that without this howler the application of intentionality to phenomenal content would never have gained currency.

Abstract: I defend the intuition that the phenomenology of our experience is right in attributing the colors we see to objects; but although colors are properties of objects, they are constitutively dependent on the perceiver’s experiences. I offer a metaphysical account for this primitivist intuition, in response to David Chalmers’ arguments against it, drawing inspiration from Aristotle’s theory of causation.

Our thesis is that proprioception is not a sixth sense distinct from the sense of touch, but a part of that tactile (or haptic) sense. The tactile sense is defined as the sense whose direct intentional objects are macroscopic mechanical properties. We first argue (against D. Armstrong, 1962; B. O'Shaughnessy 1989, 1995, 1998 and M. Martin, 1992, 1993,1995) that the two following claims are incompatible : (i) proprioception is a sense distinct from touch; (ii) touch is a bipolar modality, that (...) intrinsically has both a subjective-bodily pole and objective pole. We then argue that the bipolarity of touch should be preferred over the introduction of a sui generis sense of the body. We try to revive Aristotle suggestion according to which the body is the tactile medium (like the air for sight). Since this medium is constantly changing its shape, we need some specific channel of information about its state : proprioception, functionally defined, is that part of touch which informs us about the state of this changing tactile medium. Though muscular and articular receptors are usually dedicated to inform us about the mechanical properties of the tactile media, and the skin receptors about the mechanical properties of the tactile objects, this is not essentially so. In weighting or wielding experiments we access the weight of external objects even when skin sensitivity is absent; in prosthetic touch, the skin receptors play the role usually assigned to muscle and articular receptors, namely to inform us about the mechanical state of the tactile medium. So proprioception, anatomically defined, can play both the role of informing us about the tactile medium, or about the tactile objects. That other sensory modalities also rely on proprioceptive information should be understood in terms of cross-modal dependencies: of sight, hearing, smell, taste...on touch. (shrink)

In De Anima III.3 Aristotle presents his official discussion of phantasia (“imagination” in most translations). At the very outset of the discussion Aristotle offers as an endoxon that “phantasia is that in virtue of which we say that a phantasma occurs to us” (428a1-2). Now a natural reading of this claim, taken up by many commentators, can pose a problem for Aristotle’s overall account of perception. Here I argue that, although it would be silly to deny that Aristotle considers phantasia (...) to be that in virtue of which a phantasma occurs to someone, it is not silly to deny that phantasmata are present in every case in which phantasia is operative. This suggestion bolsters the idea that, according to Aristotle’s account of veridical perception, we can understand the world as making itself perceptually manifest to the perceiver. (shrink)

This chapter has two objectives. The first is to clarify Aristotle’s view of the first principles of the sciences. The second is to stake out a critical position with respect to this view. The paper sketches an alternative to Aristotle’s intuitionism based in part on the use of quantitative inductive logics.

Review of Johanson's book Aristotle on the sense organs. Aristotle seeks to explain the characteristics of the different sense organs by reference to the goal that they serve, that of enabling animals to perceive. A material basis is necessary for sense perception but it is an open question whether the material in question undergoes a physiological change.

The paper discusses the active part in the process of perceiving, usually expressed by the Greek word krinein. It is argued that krinein in one of its uses means "to judge" in the sense of judging a case, i. e. deciding it. It is not used for making statements. A second meaning of the Greek word is that of discerning or discriminating, and it is this meaning that plays a central part in Aristotle's theory of perception.