Difficulties for the Philosophy of Illusion

Illusion:

1.     a deceptive or misleading appearance

2.     a false or misleading impression, idea, belief or understanding

3.     a false perception of an object or experience due to the mind misinterpreting the evidence relayed to it by the senses.

The concept of illusion is an ancient one, yet its venerable pedigree and widespread popularity are no guarantee of its value as a tool for uncovering the nature of perception. It is a handy concept for sure, but perhaps we should be wary of convenience, especially as a route to insight.

Examples of illusion are not difficult to come by - many are the stock-in-trade of conjurers and ‘illusionists’, but where some illusions might be distinguished, especially from the tricks and entertaining feats of stage-artists, is in the degree to which they might be regarded as having something to reveal about the workings of perception. When a conjurer uses slight of hand to “deceive the eye”, we do not suppose that this has anything useful to tell us about our sensory capacities. The idea that, with sufficient skill and dexterity, the hand can move more expertly than is easily perceived is unsurprising. Optical illusions, on the other hand, produce puzzling responses or anomalous visual artefacts that call for more sophisticated explanations.

Philosophers – knowing that their theories often stand or fall on the evidence of scientific enquiry – frequently refer to optical illusions in order to substantiate their claims. However, during the 1960’s, an important body of evidence emerged that cast significant doubt on many of these claims, yet has gone largely unacknowledged in philosophical circles.

Müller-Lyer Illusion

In a 1966 study undertaken by Segal et al into cross cultural variations in susceptibility to optical illusions the researchers found significant variance between differing communities and age groups across the globe. Some groups, for instance, reported little or no difference between the apparent lengths of the lines of the famous Müller-Lyer diagram. An earlier study by Hudson (1960), of culturally isolated South African children, encountered very similar findings. Both studies attributed their results to a lack of habitual exposure to pictures amongst the communities studied. Hudson dubbed this lack of familiarity: ‘pictorial illiteracy’. In fact, even children well schooled in language and arithmetic skills (but lacking pictorial literacy) were not susceptible to what is commonly described as the ‘pictorial illusion of depth’ and were therefore unsusceptible to the depth cues that many optical illusions exploit.

In 2006 Robert N. McCauley and Joseph Henrich write:

For those who experience it, the illusion may persist, but susceptibility to the Müller-Lyer illusion is neither uniform nor universal. Moreover, a plausible argument can be made that through most of our species’ history most human beings were probably not susceptible to the illusion.

If this is correct, and corroborating evidence can be provided from the art historical record to support it - then there is good cause to doubt the relevance of illusion in the explanation of perception. Moreover, if the following evidence is anything to go by, these doubts are in greater need to explanation than ever.

In numerous well documented studies, it has been shown that when people reach to grab three-dimensional versions of optical illusions, their grip aperture (the distance between finger and thumb) is unaffected by the illusion. So, whilst we may be inclined to say that one part of an optical illusion appears to be larger than the other, our ability to physically interact with these illusions is unaffected.

From an evolutionary point of view, it is of the utmost importance that we do not confuse a distant object for a small object, especially if the distant object has significance for our potential to survive. It is extraordinarily fortunate in fact, that the capacity to recognise and use perspectival images (in which distant objects are depicted at disproportionate scales than nearby objects) has not been entirely overridden by the evolution of our perceptual skills. If the research of Hudson, Segal et al is correct, then it would seem that this capacity to derive depth cues from perspectival images is a learnt skill and is not an immediately available part of our genetically acquired perceptual repertoire. And McCauley and Henrich are surely right when they speculate that our susceptibility to illusions must be a relatively recent consequence of the increasingly widespread use of pictorial imagery. What better explanation do we have for the widespread indifference amongst animals to our attempts to interest them with images?

"The Innocent Eye Test", Mark Tansey, 1981

I hope the evidence presented here makes it clear that the standard view of illusion - as deceptive, misleading or false - is thoroughly inadequate as a tool for the investigation of the nature of perception.  If our theories don’t fit the evidence then it is time to change our theories. I suggest that we start with the theory of illusion.