Perceptual objects. Argument from illusion Author: Elena Pasquinelli [INSTNICOD] Contributors: none Current version (on 2006-01-23) The development of enactive interfaces based on action and perception has the effect of bringing the attention on the functioning of perception. Since enactive interfaces have the aim of producing perceptual objects and meaningful perceptual interactions, a relevant problem is constituted by the problem of what counts as an object of perception. An overview of different possible answers to this problem is presented here in relationship to the so-called ‘argument of illusion’, where the experience of reality is compared to the experience of illusions. The argument from illusion (see [Ayer, 1955] for its classical formulation) can be schematized as follows [Dokic, 2004]: all experiences have an object, but the experience of illusions lacks a material object. The objects of experiences are all the same, both for illusory and veridical experience, therefore the objects of experience are not material objects. The immaterial objects that are supposed to be directly perceived in illusory and non-illusory experiences are the sense-data. In the case of the stick that looks bent, for instance, the experience of the pretended illusion is assimilated to the experience of a delusion, which lacks of any reality. Thus, since no real object is perceived, but some kind of object must be, the existence of immaterial objects or sense-data is postulated. [Austin, 1962] opposes two main criticisms to the argument from illusion. First, the argument from illusion is based on a wrong definition of illusion. Illusions are different from delusions and from familiar mistakes and unusual perceptual phenomena. The class of illusions only includes public, reproducible and surprising phenomena such as the geometric illusions or the tricks of the magician. Second, it is not strange or surprising that an object that is in a certain way, looks in another, in special conditions. Thus there is no need for postulating special objects that are directly perceived: what we perceive are the ordinary things.

Different positions can be traced in respect to the argument from illusion that belong to different views of perception and illusions [Dokic, 2004]. The argument from illusion is defended by phenomenists and the indirect realists and it is rejected by three theories of perception: the disjunctive theory, the bipolar theory and the adverbial theory. Phenomenism sustains that all we can perceive are sense-data and that perception does not regard objects that are external and independent from the perceiver. According to indirect realism perception can only give access to sense-data, but reality is not limited to sense-data: the physical world exists and can be known because of the structural, causal relations between physical reality and sense-data. The adverbial theory considers that the so-called objects of perception are in reality modifications of the verbs of perception, as adverbs are. Hence the distinction between veridical experiences and illusions depends on the fact that veridical experiences are appropriately caused by elements of the physical reality. The bipolar theory considers that perception does not consist of the experience and of the intentional object only, but also of propositional contents of perception. Illusions and veridical experience can thus have the same content but not the same object (only veridical experiences have an object). In the disjunctive theory of perception illusions and veridical perception are considered as two different phenomena. When one is fooled by the world one is not perceiving a fact of the world, but he is just having a perceptual experience. Perceptual experiences, or the mental states that are described by the verb “having the perceptual experience that p”, can both be veridical or illusory; but the two states, are different in their essence, because only the first one has for object a state of the world..

References: Austin, J. L. (1962). Sense and sensibilia. London, Oxford, New York: Oxford University Press. Ayer, A. J. (1955). The foundations of empirical knowledge. London: MacMillan and Co. Dokic, J. (2004). Qu'est-ce que c'est la perception? Paris: Vrin.

Related items: Illusion Perception. Direct and indirect approaches Active perception

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