Sensation and Perception-Perceptual Illusions.

The objects of intentional states are sometimes called“intentional objects” (Crane (2001: Chapter 1)). What arethe intentional objects of perceptual experience, according tointentionalists? In the case of veridical perception, the answer issimple: ordinary, mind-independent objects like the churchyard, thesnow (etc.) and their properties. But what should be said about thehallucinatory case? Since this case is by definition one in whichthere is no mind-independent object being perceived, how can we eventalk about something being an “object of experience” atall here? As noted above, intentionalists say that experiences arerepresentations; and one can represent what does not exist (see Harman(1990), Tye (1992)). This is certainly true; but isn’t there anymore to be said? For how does a representation of a non-existentchurchyard differ from a representation of a non-existent garbagedump, say, when one of those is hallucinated? The states seem to havedifferent objects; but neither of these objects exist (see the entry).

Just take a peek at the Perceptual Illusions on last page of this report.

Where the argument from hallucination is controversial is in thespreading step. The spreading step here gets construed in terms of theidea that veridical experiences and hallucinations are essentially thesame; mental events of the same fundamental kind (Martin(2006)). (This doesn’t mean that we lose a distinction betweenveridical experiences and hallucinations. It just means that thedifference between veridical experience and hallucination is not to befound in their intrinsic natures). This claim seems plausible, as froma subject’s perspective an hallucination cannot be told apartfrom a veridical experience. Thus some will accept (B) and thus denythat we are ever perceptually aware of ordinary objects (seesense-datum theories ), and others willaccept (B) but argue that we can still have perceptual awareness ofordinary objects (see intentionalist theories ). But as we’ll see, others will wantto secure perceptual awareness of ordinary objects by rejecting (B)and holding that hallucinations and veridical experiences arefundamentally different (see naive realist theories and disjunctivism).


This development of perception could be learnt or innate.

Rather, human perceptual experience grants us only partial access to the full range of properties that such objects possess. That some of these properties, like colours, turn out to be relatively complex does not make them any less real. Instead, the scientific and commonsense world views each capture different aspects of reality at different levels of abstraction or description. Far from our perceptual experience being illusory, then, as scientists sometimes claim, it is the idea that these two levels of description are necessarily incompatible or mutually exclusive that is the true, more subtle illusion.


Perception is the ability to see, ..

The most controversial premise in the argument is premise (ii). Theother premises just reflect intuitive ways of thinking aboutperceptual experience, and so are unlikely to be targeted by oneseeking to reject the argument from illusion. This is clear enoughwith premises (i) and (vi), but what about premise (iv)? What thismeans is that the account of the nature and objects of illusory andveridical experiences must be the same. Though it may be disputed,this premise seems plausible. For veridical and illusory experiencesboth seem to be cases where one is aware of an ordinary object. Theonly difference is that in the illusory case, but not in the veridicalcase, the object one is aware of appears some way other than it infact is.

Color is also a major variable in testing perception after illusions.

The problem, then, is not that reality contains no colours, but rather that it contains too many, both within and beyond the range of electromagnetic radiation to which our eyes are sensitive. Many of these exotic look identical or are completely invisible to us, making our perceptual abilities limited at best. Does this mean that the colours we are capable of seeing are somehow unreal? Not necessarily. To understand why, let us consider a different example: the notion of solidity.

who considered the perception of illusions, ..

According to Awareness, we are sometimes perceptuallyaware of ordinary mind-independent objects in perceptualexperience. Such awareness can come from veridicalexperiences—cases in which one perceives an object for what itis. But it can also come from illusory experiences. For we think of anillusion as “any perceptual situation in which a physical objectis actually perceived, but in which that object perceptually appearsother than it really is” Smith (2002: 23). For example, a whitewall seen in yellow light can look yellow to one. (In such cases it isnot necessary that one is deceived into believing that things areother than they are). The argument from illusion, in a radical form,aims to show that we are never perceptually aware of ordinaryobjects. Many things have been called “the argument fromillusion”. But the basic idea goes as follows: