Editing Direct and indirect realism (section)

===Argument from illusion===
[[Image:Café wall.svg|thumb|upright=1.15|Illusion creates a problem for naïve realists as it suggests our senses are fallible, perceiving things that are not there. In this illusion, the lines are horizontal, despite how they appear.]]
This argument was "first offered in a more or less fully explicit form in [[George Berkeley|Berkeley]] ([[Three Dialogues between Hylas and Philonous|1713]])."<ref name="sep" /> It is also referred to as the problem of conflicting appearances (e.g. [[Myles Burnyeat]]'s article ''Conflicting Appearances''). It has been argued that "informed commonsense" indicates that perceptions often depend on organs of perception.<ref name="uor">[http://www.rdg.ac.uk/AcaDepts/ld/Philos/jmp/Theory%20of%20Knowledge/NaiveRealism.htm Naïve Realism] {{Webarchive|url=https://web.archive.org/web/20091002080457/http://www.rdg.ac.uk/AcaDepts/ld/Philos/jmp/Theory%20of%20Knowledge/NaiveRealism.htm |date=2009-10-02 }}, [[University of Reading]].</ref> For example, humans would receive visual information very differently if they, like flies, had compound eyes, and may not even be able to imagine how things would appear with entirely different sense organs such as [[infra-red]] detectors or [[acoustic location|echo-location]] devices. Furthermore, perception systems can misrepresent objects even when in full working order, as shown, for example, by [[optical illusion]]s like the [[Müller-Lyer illusion]]. More dramatically, sometimes people perceive things which are not there at all, which can be termed instances of "hallucination" or "perceptual delusion".<ref name="uor" />

[[Image:Double-alaskan-rainbow.jpg|thumb|upright=1.15|Illusions are present in nature. [[Rainbows]] are an example of a perceptual delusion. "For, unlike an architectural arch, a rainbow recedes as we approach it, never to be reached."<ref>[[Richard Gregory|Gregory, Richard.]] (2003). [http://www.perceptionweb.com/perception/perc0303/editorial.pdf Delusions.] {{Webarchive|url=https://web.archive.org/web/20110715064230/http://www.perceptionweb.com/perception/perc0303/editorial.pdf |date=2011-07-15 }} ''Perception.'' '''32''', pp. 257-261.</ref>]]

The [[argument from illusion]] allegedly shows the need to posit sense-data as the immediate objects of perception. In cases of illusion or [[hallucination]], the object has qualities that no public physical object in that situation has and so must be distinct from any such object.<ref name=sep/> Naïve realism may accommodate these facts as they stand by virtue of its very vagueness (or "open-texture"): it is not specific or detailed enough to be refuted by such cases.<ref name=uor/> A more developed direct realist might respond by showing that various cases of misperception, failed perception, and perceptual relativity do not make it necessary to suppose that sense-data exist. When a stick submerged in water looks bent a direct realist is not compelled to say the stick actually is bent but can say that the stick can have more than one appearance: a straight stick can look bent when light reflected from the stick arrives at one's eye in a crooked pattern, but this appearance is not necessarily a sense-datum in the mind. Similar things can be said about the coin which appears circular from one vantage point and oval-shaped from another. Pressing on your eyeball with a finger creates double vision but assuming the existence of two sense-data is unnecessary: the direct realist can say that they have two eyes, each giving them a different view of the world. Usually the eyes are focused in the same direction; but sometimes they are not.

However, this response is presumably based on previously observed data. If one were to be able to observe nothing other than the stick in the water, with no previous information, it would appear that the stick was bent. Visual depth in particular is a set of inferences, not an actual experience of the space between things in a radial direction outward from the observation point.<ref>Green, Alex. (2003). [http://www.users.globalnet.co.uk/~lka/conr.htm The Empirical Description of Conscious Experience] {{Webarchive|url=https://web.archive.org/web/20110805095730/http://www.users.globalnet.co.uk/~lka/conr.htm |date=2011-08-05 }}, ''The Science and Philosophy of Consciousness.''</ref> If all [[empirical evidence]] is based upon observation then the entire developed memory and knowledge of every perception and of each sense may be as skewed as the bent stick. Since objects with different qualities are experienced from each of the different perspectives there is no apparent experiential basis for regarding one out of any such set of related perceptual experiences as the one in which the relevant physical object is itself immediately experienced. The most reasonable conclusion is that the experienced object is always distinct from the physical object or at least that there is no way to identify which, if any, of the immediately experienced objects is the physical object itself. Epistemologically it is as though physical objects were never given, whether or not that is in fact the case.<ref name=sep/>

Another potential counter-example involves vivid hallucinations: phantom elephants, for instance, might be interpreted as sense-data. A direct realist response would differentiate hallucination from genuine perception: no perception of elephants is going on, only the different and related mental process of hallucination. However, if there are visual images when we hallucinate it seems reasonable that there are visual images when we see. Similarly if dreaming involves visual and auditory images in our minds it seems reasonable to think there are visual and auditory images, or sense-data, when we are awake and perceiving things. This argument has been challenged in a number of different ways. First it has been questioned whether there must be some object present that actually has the experienced qualities, which would then seemingly have to be something like a sense-datum. Why couldn't it be that the perceiver is simply in a state of seeming to experience such an object without any object actually being present? Second, in cases of illusion and perceptual relativity there is an object present which is simply misperceived, usually in readily explainable ways, and no need to suppose that an additional object is also involved. Third, the last part of the perceptual relativity version of the argument has been challenged by questioning whether there is really no experiential difference between veridical and non-veridical perception; and by arguing that even if sense-data are experienced in non-veridical cases and even if the difference between veridical and non-veridical cases is, as claimed, experientially indiscernible, there is still no reason to think that sense-data are the immediate objects of experience in veridical cases. Fourth, do sense-data exist through time or are they momentary? Can they exist when not being perceived? Are they public or private? Can they be themselves misperceived? Do they exist in minds or are they extra-mental, even if not physical? On the basis of the intractability of these questions, it has been argued that the conclusion of the argument from illusion is unacceptable or even unintelligible, even in the absence of a clear diagnosis of exactly where and how it goes wrong.<ref name=sep/>

Direct realists can potentially deny the existence of any such thing as a mental image but this is difficult to maintain, since we seem able to visually imagine all sorts of things with ease. Even if perception does not involve images other mental processes like imagination certainly seem to. One view, similar to Reid's, is that we do have images of various sorts in our minds when we perceive, dream, hallucinate and imagine but when we actually perceive things, our sensations cannot be considered objects of perception or attention. The only objects of perception are external objects. Even if perception is accompanied by images, or sensations, it is wrong to say we perceive sensations. Direct realism defines perception as perception of external objects where an "external object" is allowed to be a photon in the eye but not an impulse in a nerve leading from the eye. Recent work in neuroscience suggests a shared ontology for perception, imagination and dreaming, with similar areas of brain being used for all of these.

The character of experience of a physical object can be altered in major ways by changes in the conditions of perception or of the relevant sense-organs and the resulting [[Neurophysiology|neurophysiological]] processes, without change in the external physical object that initiates this process and that may seem to be depicted by the experience. Conversely any process that yields the same sensory/neural results will yield the same perceptual experience, no matter what the physical object that initiated the process may have been like. Furthermore, the causal process that intervenes between the external object and the perceptual experience takes time, so that the character of the experience reflects, at the most, an earlier stage of that object than the one existing at the moment of perception. As in observations of astronomical objects the external object may have ceased to exist long before the experience occurs. These facts are claimed to point to the conclusion that the direct object of experience is an entity produced at the end of this causal process, distinct from any physical object that initiates the process."<ref name="sep">[http://plato.stanford.edu/entries/perception-episprob/ Epistemological Problems of Perception] {{Webarchive|url=https://web.archive.org/web/20110525001656/http://plato.stanford.edu/entries/perception-episprob/ |date=2011-05-25 }}, Stanford Encyclopedia of Philosophy.</ref>