I Refute It Thus!

Some theorists take the view that we have no direct perceptual access to the world. They argue that perception is mediated by our representational skills, creative techniques and—if they are to be consistent—the raw materials we use as well (although—tellingly—many would deny this latter condition). This doctrine is known as Transcendental Idealism and was first propounded by the German philosopher Emanuel Kant in the 18th Century. Idealism also comes in a vanilla edition which takes perception to be a creation of the mind or brain for the benefit of... well for the benefit of the mind or brain I guess. Many withering and sometimes funny attempts have been made to discredit Idealism, but its followers seem to be incurable. In contrast to both forms of Idealism, Realism—which also comes in various flavours—takes the world to be very largely as we find it.

The Realist philosopher Karl Popper claimed that Idealism (including Transcendental Idealism no doubt) and Realism are “neither demonstrable nor refutable”. Perhaps he was right. However, some would argue—with a tinge of irony—that his claim itself is not beyond refutation. One famous attempt at a refutation of Idealism was performed by Samuel Johnson, who kicked a neighbouring stone and quipped: “I refute it thus!” Most philosophers find his demonstration to be thoroughly unconvincing. Nonetheless, Johnson’s perfunctory gesture may have more to commend it than is ordinarily conceded or acknowledged.

Natural or manufactured objects, like the heart [or brain], chemical agents, the planets or engines, have an action, which may be slow, complicated or beautiful; but they do not take action, they do not act, however much they may act on other things. (Alan White "The Philosophy of Action", 1968 p.2)

Johnson had to take action to kick the rock; in order to make his demonstration. There is no such thing as a representation without some actions being taken to produce it. But a brain or mind cannot take action, least of all representational action. Brains are obviously significantly involved in the taking of actions but they do not have agency or take actions on their own or anyone else's behalf. The Idealist's claim that the mind/brain creates representations simply doesn't stand up to scrutiny. Even Transcendental Idealists reject Idealism as wholly incoherent.

When we act, our actions are comprised of countless unthinking physiological processes that have been shaped by millions of years of evolutionary development. Representation is merely a very recent cultural and fully public outgrowth from a winnowing process that has left countless behaviours and sensory failures in its wake. Life in general is a testament to the undeniable efficacy of mindless sensory responsiveness. It is this sensory integration, and not our representations or even our perceptions, that determines what we reliably take for granted.

“The thesis that ‘our representational practices determine all the divisions’ is vulnerable to the criticism that the de facto success of our representational practices can only be attributable to regularities that are implicit in the relationships between the components of the universe itself.” (Donald Brook, in personal correspondence 20/09/16)

Some people find Brook a little difficult to grasp, so perhaps I can try to put it differently. If the world were not comprised of objects and circumstances in precisely the configurations that we find them, then our representations, not to mention our biological processes, would never have gotten off the ground in the first place.

Illusionistic Innovations

If you see the drawing as such-and-such an animal, what I expect from you will be pretty different from what I expect when you merely know what it is meant to be. (L. Wittgenstein. Philosophical Investigations. 1953, p205e)

If you merely know that a young child’s drawing is meant to be of a cat, it is probable that the drawing shares very little in common with a cat. But if you momentarily mistook a photograph or skilful drawing of a cat for an actual cat, it would be extraordinary if the picture turned out to share nothing in common with a cat.

Now a simple phenomenological inspection of any representation, either a drawing or a photo, shows us that an image possesses none of the properties of the object represented. (U. Eco. “Critique of the Image,” 1970)

For Eco, the relation between images and “real phenomena” is “wholly arbitrary”. But this is surely mistaken. Words like “cat” certainly do have a "wholly arbitrary" relation to the things they refer to. Consequently there is no question of our mistaking the word “cat” for a four-legged animal of the feline variety. But if, as Eco claims, images also share “none of the properties” of the things they represent, then how is it possible that we can very occasionally mistake what turn out to be images for the things they represent? Eco offers no explanation. I suggest that if it is true that we can sometimes mistake the properties of one thing for the wholly different properties of another thing, then it is reasonable to suppose that we must be dealing with some form of illusion.

It is important to realise here how familiarity, so to speak, takes the edge off illusion. Is the cinema a case of illusion? Well, just possibly the first man who ever saw moving pictures may have felt inclined to say that here was a case of illusion. But in fact it's pretty unlikely that even he, even momentarily, was actually taken in; and by now the whole thing is so ordinary a part of our lives that it never occurs to us even to raise the question. We might as well ask whether producing a photograph is producing an illusion—which would plainly be just silly. (J. L. Austin. “Sense and Sensibilia.” 1960, p26)

Silly as it might be to ask such a question, it wouldn't be silly to suppose that we could use a photograph to construct an illusion. Nor would it be silly in certain circumstances to momentarily mistake a life-sized photograph of a person for an actual person. So whilst familiarity may take the edge off illusion, it doesn’t eliminate the possibility of illusion and nor might it diminish the importance of the concept.

“The application of the concept of an illusion in general presupposes a concept of being wrong in the sense that were we never wrong in what we perceive, were we never to make a false judgement about what we perceive, we should not have the concept of an illusion.” (D. W. Hamlyn. Sensation and Perception. 1961, p196)

A few years earlier, Hamlyn (no relation by the way) made some similar remarks on the same subject:

There is not necessarily anything about an illusion which tells us that it is one, for if there were it would not be appropriate to say that we were ever taken in. This is not to say that it is always right to say of someone who sees something wrongly that he is taken in; for he may see it in this way despite the fact that he knows the thing in question is not like this. But in order for it to be appropriate to talk of illusions it must sometimes be the case that people are deceived.” (D. W. Hamlyn. “The Visual Field and Perception. 1957, p112)

Now if this person “knows the thing in question is not like this” then he is not seeing it “wrongly” at all. He is seeing it as anyone else with the same perceptual faculties would see it. Moreover he sees that it is illusory in some respects and he probably also knows that other people would find it illusory in the same ways. So if I say that the moon looks like a proximate flatly presented silvery circle, I am not making a perceptual claim that should be regarded as an instance of false or wrong perception. I am using a commonplace expression that will usually be readily accepted by anyone familiar with the ways in which illusory appearances are generally expressed in language.

Shared discriminatory capacities are a precondition for shared concepts of colour, taste, sound, smell, etc. Moreover, shared propensities for perceptual illusion are a precondition for shared concepts of perceptual appearances as distinct from actualities, viz. concepts of objects publicly looking thus-and-so although not being so. (Baker and Hacker. Wittgenstein: Rules, Grammar and Necessity. 2010, p.215)

One of my principal aims in exploring these issues is to show how these “concepts of perceptual appearances” are linguistic outgrowths from innovations in illusionistic representation. In other words, without these techniques, it would make no sense to say that yonder house looks small or that a glossy surface looks wet or that a static white cinema screen looks like a multi-coloured window onto a world of moving objects and people.

It may be that people without any experience of pictorial simulation would not say that the distant hills look blue, but even such innocents would probably be tricked, by being smuggled into a good planetarium, into believing that they were looking at the open night sky. (D. Brook. “How to Draw the Curtains.” 1985)

Masters of Illusion

A Cartesian can believe that the existing world is not visible, that the only light is that of the mind, and that all vision takes place in God. A painter cannot grant that our openness to the world is illusory or indirect, that what we see is not the world itself, or that the mind has to do only with its thoughts or with another mind. (Maurice Merleau-Ponty 1964, 186)

The following discussion aims to show that our perception of the world is in no way illusory. Whilst we are obviously susceptible to illusions, our various skills and tools enable us to recognise and exploit many of these susceptibilities in a variety of powerful ways.

According to Alex Byrne: "Perception comprises, by stipulation, veridical perception and illusion" (2009). For Byrne, and many other philosophers, one “veridically perceives an object if and only if one sees it, and it is the way it appears or looks.” In other words, veridical perception is what we usually mean when we speak of perception in ordinary language. It is the perception of how objects ordinarily are, or what we typically call “ordinary perception”, “normal perception” or just plain “perception”. However, like Byrne, some philosophers claim that perception “comprises”, or at least sometimes involves, illusion or illusory perceptions. Two important points need to be made regarding this claim. Firstly, an illusory parrot is not a species of parrot—in fact it is not a parrot at all. And by the same token, an illusory perception is not a perception either, at least not in the respects in which it is illusory. The same is true of misperceptions, false perceptions or perceptual mistakes etc. A failure to perceive something in certain respects cannot be a sort of perception in those respects. Missing a train is not an instance of catching a train.

Secondly, to notice or recognise an illusion is not to be at its mercy. It is to grasp the illusion for what it is. To say that one length of an optical illusion "looks" shorter than the other is not to say that the line is shorter than the other. Recognising that the two lines could be mistaken as being of different lengths does not preclude our seeing that they are the same length.

In a forthcoming book chapter, John O’Dea writes: "I can think of no good reason to deny that a tilted coin could be seen as elliptical and flat with respect to the viewer. This would be tantamount to denying the possibility of illusion." It is perfectly justified to say that a tilted circular coin can be treated, regarded or considered as a flatly presented ellipse, because a tilted circular coin can be successfully depicted as a flatly presented ellipse. However, if O’Dea intends “seen” in the sense of “perceived”, which seems likely in the context of his discussion, then his claim should be examined in light of the conclusions we have already drawn about the relation between perception and illusion. Thus, if the alleged perception is an illusion, then it is not a de facto perception in the relevant respects. On the other hand, if looking at a tilted circular coin results in a perception of an illusion—in seeing the illusion for what it is—then a description of the illusion alone will not answer the question of what has been perceived. What has been perceived is a tilted circular coin that can be successfully depicted through the use of a flatly presented ellipse. 

In the concluding paragraph of the chapter, O’Dea writes:

Constancy often fails; deep shadows can make surface colour perceptually unclear; at severe angles, shapes constancy disappears; size becomes harder to judge from more distant objects; and so on. Is perceptual experience illusory in these conditions?

Evidently O’Dea has mistaken illusions of inconstancy for actual failures of constancy. Constancy is usually characterised as the stability we regularly encounter in the properties of perceived objects, despite changes in angle of view, illumination, shading etc. So if circumstances of apparent inconstancy do not constitute actual inconstancy, but rather the illusion of inconstancy, then it would be false to conclude that they are perceived as inconstant. A failure to see that a dead parrot is deceased is not a failure of constancy on the part of the parrot.

If the properties of a stable object, like a book or a table (or even a dead parrot), are perceived as having constancy, there is nothing to prevent us from also regarding, considering or treating these same objects as if they have inconstant features like being blurred when viewed at close quarters or being small when seen from a distance or being colourless in moonlight etc. This is important because it shows that we are often capable of treating things in two quite different ways, one of which involves the capacity to represent the actual properties we perceive whilst the other involves a more sophisticated knowledge of how to represent objects by way of illusionistic representational techniques.

The following passage from Maurice Merleau-Ponty makes a related point:

It took centuries of painting before the reflections upon the eye were seen, without which the painting remains lifeless and blind, as in the paintings by primitive peoples. The reflection is not seen for itself, since it was able to go unnoticed for so long, and yet it has its function in perception, since its mere absence is enough to remove the life and the expression from objects and from faces. [...] It is not itself seen, but makes the rest be seen. Reflections and lighting in photography are often poorly portrayed because they are transformed into things..." (2012, §364)

It is true that eyes quickly lose their lustre in the absence of lubrication, but if the reflection is not seen for itself, an obvious question arises. Is the reflection on a pond seen for itself, or the reflection in a mirror? Merleau-Ponty provides no obvious answer, but it seems reasonable to conclude that his view would be this. Until the discovery of depictive techniques capable of transforming reflections into things, people were not capable of representing them. In other words, it is by virtue of the emergence of pictorial techniques that visual phenomena like blurring, reflections, perspectival distortions etc. have become communicable (there is even evidence that this applies to the colour blue). This is not to say that reflections have not always played a part in perception. After all, if tears produced no reflections, they would be invisible. As Merleau Ponty says: the reflection “is not itself seen” but it enables the tears, the pond etc. to “be seen”. And what of mirrors? Do we not perceive the reflection in a mirror? His point is that the reflection is not perceived “for itself” as a thing. We might occasionally mistake a reflection for a thing, but this would not constitute a perception. It would be the perceptual equivalent of sitting on the platform of Glasgow Queen Street station dreaming that you are on the 8:15 to Edinburgh Waverly.

Schwitzgebel (2011) raises another reason to be wary about the claim that perception involves illusion. 

Consider the oar's looking bent in water. Could we say that the oar's appearance is an illusion? That seems natural. But if so, then presumably the look of things through a glass of water, which will be similarly distorted, is also an illusion. And if that, then also the look of things through a magnifying glass held appropriately close? Through a telescope? Through ordinary corrective lenses? (166n.8)

It makes little sense to say, for example, that when things recede into the distance, they gradually become illusory. They are simply harder to make out. When we look across a table, we see less of the far side. This isn't an illusion. It's a commonplace and unremarkable consequence of the spatial fall-off of sensory input. The more distant an object, the less we see of it. The effect is regular and gradual and leads eventually to the complete loss of input as objects recede into the distance. If our sensory systems were perfect, there would be no fall-off. But then again, if our sensory systems were perfect, there would be nothing we could not perceive and there would be no such thing as partially seeing or barely hearing something etc. Partial perception and the gradual failure to make things out in certain respects are just normal characteristics of our sensory relation to the world.

In ordinary circumstances, when we perceive things normally, we often describe them as if they have properties that they do not actually possess. We might say that shiny surfaces commonly "look silvery or wet", that rainclouds "look leaden or grey" or that fast-moving objects "appear to be blurred". These are not perceptual reports (or at least not perceptual success reports), but they do not preclude other (arguably more objective) ways of reporting things in terms of their actual perceived properties. In normal usage though, it doesn't really matter which strategy of representation we invoke in describing the objects we see (or "seem to see" on account of the ordinary fallibilities of all sensory systems), because we all familiar with the same representational techniques and we are all subject to very much the same perceptual strengths and weaknesses.

Very much less theoretical attention has been paid to those perceptual failures that are the logical corollary of success. […] In the wake of each positive perceptual advance the reciprocal logic of discrimination failure opens up new prospects for influential representational substitution. (Brook 1997)

This is a vitally important observation with profound implications. To be under the spell of an illusion is to be incapable either of recognising it or exploiting it. It is simply to make a mistake. However, to recognise an illusion is not only to be capable of recognising and perhaps avoiding other similar illusions. It is to have an insight into how other similar illusions might be staged and used as effects. While many creatures are capable of learning from their mistakes, only tool-users are capable of recognising and exploiting their mistakes in the form of illusionistic effects.

Visibilia and Perceptibilia

Perhaps the best way to introduce visibilia is through a novel but commonplace example. When we view someone in profile, we see a person who appears to have only one eye. If we were to produce an observational drawing of what we see, it would be inaccurate to render them with two eyes where only one is visible. So perhaps in this sense we might be forgiven if, on consideration, we became a little confused about how best to describe what we actually perceive.

Essentially visibilia are appearances in which things of one kind look-like things of another kind. A question that plagues contemporary philosophy of perception and perceptual psychology is whether there might be any truth to the claim that we actually perceive visibilia, whether they have any causal influence on us or whether, on the contrary, they are non existent figments. Several other questions arise. If visibilia are non-existent, then how did we ever come to conceive of them in the first place or to mistake whatever it is that we do perceive for them? And don’t photographs categorically prove the existence of visibilia? Aren’t visibilia out there in the world in a measurable quantifiable sense and if so then surely we perceive them? Surely visibilia have influence upon us?

Michael Martin (2010) argues that mirror images, rainbows, holograms and even the sky should be included in the class of visibilia. We might even say these things are more visibilia than perceptibilia – the contrast will turn out to be informative.

Many keyboards have been tapped into oblivion over the questions of visibilia yet there seems to be no easy resolution to the problem. However, perhaps rather than focusing on examples like mirrors and rainbows etc. it might be more revealing to dwell a little on the more absurd example I gave at the outset. If it is true that we do indeed perceive a one-eyed person when viewed in profile, then this sight should surely be a little surprising. If we could conduct a brain scan of someone as they watch another person rotating, there should be peaks of arousal when the observer sees only one eye and perhaps yet more arousal when they see no eyes at all—as the figure turns to face away from the viewer.

I would be willing to wager a considerable sum that there wouldn’t be any appreciable arousal at all. In fact I suspect that the level of arousal might actually diminish as any face is turned away from us.

I’m not at all convinced that we perceive visibilia, and the reason I’m not convinced is because perception is in no way an impoverished snapshot of the world. Perception, at every moment, is interlaced with knowledge about the things that we see. We have a confidence bordering on certainty that the person moving before us is a three dimensional being. And in the instant that we see them we are host to a profusion of latent expectations about what we would be likely to see as they move around etc. These expectations do not suddenly come into being in the moment of encounter. They are merely part of our repertoire of possible responses to the world: our knowledge. As someone spins around before us the perceptible properties constantly update and confirm what we already know and, whenever they do not confirm what we know, it is then that we should expect to register some form of surprise because we now see something unexpected.

Black from White

When I was around the age of seven, my parents had close friends who lived a few doors away from Camberwell School of Art, where I think they had studied some years earlier. They had a daughter, Esmie, who was three years my junior and I remember she once said something that struck me as obviously wrong - she claimed that white was not a colour. I tried to disabuse her of this misconception only to be gently corrected by our dads. I don't recall their explanation or if they even gave one but ever since I have always harboured a distrust of white.

Strictly speaking, Esmie was right that white is not a colour, at least not a chromatic colour. Look in the Chambers English Dictionary though, and the first entry you will find states: "White /wīt or hwīt/ adj of the colour of snow, the colour that reflects the maximum and absorbs the minimum of light rays." As we will see, getting to grips with white, turns out to be a little more difficult than might at first seem the case. White, as it turns out, is a bit of a grey area.

When the dads stepped into my dispute with young Esmie over the subtleties of colour definitions, perhaps they responded with something like: "White is actually a combination of all the colours." That, and the fact that I was obviously in the minority, might have shut me up for a while but a quick visit to a set of poster paints would soon have demonstrated the inadequacy of this explanation. No amount of deft colour mixing—at least of the kind that I was familiar with—would ever have generated white. Why otherwise was there a block of white in all the sets of paint I had ever owned if not for the fact that it is impossible to concoct such a non-colour from a combination of all the colours? Were my parents trying to taunt me with my obvious inability to blend colours in precisely the right formulation? Was the art of colour combination simply too sophisticated for a mere child equipped with a set of cheap paints? When in later childhood I discovered that watercolour sets standardly include no white, my worst suspicions seemed confirmed. Quite how the four-year-old Esmie could have understood such complexities escapes me. Perhaps her collection of felt-tip pens revealed a deeper secret.

Of course the answer to this conundrum lies in the distinction between subtractive and additive techniques of colour-mixing. Dyes or pigments are the components of the subtractive system because, when mixed, they subtract from white or—strictly speaking—they absorb more wavelengths of light. The additive system on the other hand mixes coloured light - the equal combination of red blue and green (yes green) combining to form white (for humans that is).

So far so good, whiteness consists of the combination of equal quantities of visible spectra. But the problems really only start here because although we have established that white is not a chromatic colour, we haven't yet established much else besides. Indeed, if it weren't for the fact that we can easily select instances of white in the world then the challenge of exemplifying the concept would be almost impossible. One possible strategy might be to say that white is the opposite of black. This is helpful but we're not out of the woods yet, far from it, in fact we're heading straight into a thicket and by the time we're through to the other side I hope to have convinced you that, in certain circumstances and without changing its chemical composition or intensity at all, white can become a perfectly serviceable simulation of black.

All we need for this thought experiment is a dimmish room, a white screen and a projector. So long as the environment isn't too bright then we should be able to see any image cast into the screen by the projector. First imagine that the projector is off. Every normally sighted human capable of expressing a rational opinion about what they see, would describe the screen in this state as white. For the sake of accuracy let's also check the intensity of light reflected from the screen to determine that it is currently reflecting an exposure value (EV) of 5 which is perfectly good for reading etc. but isn't too bright for our purposes.

Now let's turn the projector on and cast a blank rectangle of light into the screen. The physical properties of the screen haven't changed so it seems likely that everyone would still agree that the screen remains white—though the illuminated parts are now obviously brighter than the unlit parts. If we take another light-measurement of these dark portions of the screen we will find that they remain at an EV of 5 (i.e. what everyone agreed was white).

Now, let's project a simple greyscale image into the screen, an image of a room with black and white floor tiles and let's ask the audience whether the tiles look black and white. Once again everyone will agree - the tiles in the image look black and white. We can also take another light-measurement of the black tiles and once again we will find that it remains at 5, or even possibly slightly brighter.

So, what's going on? How can an objectively white thing seem so uncontentiously black? Philosophers and psychologists, like well-meaning dads, are tempted to jump in at this point to inform us that the phenomenon is the result of an illusion in which we misjudge, misread or miscalculate the white as black. Essentially they take the view that illusion is dependent upon a failure of cognition of some kind. Failure does indeed play an instrumental role in the explanation of what is going on, but the failure is definitely not one of reason (judgement), literacy (reading) or computation (calculation), nor in fact is it a failure of perception: it is a failure of sensory discrimination.

So, let's set the question of illusion aside here and start instead with a simple explanation of sensory discrimination. All organisms are engaged with the world by means of their senses. If senses are triggered, organisms respond. Brains then, are part of sophisticated systems of responsiveness in which organisms possess both genetically acquired and learned responses to the things they encounter. Some stimuli will inevitably be more important than others and it will therefore be in each organism's interests to respond advantageously to relevant stimuli and to be unresponsive to irrelevant stimuli. Sensory discrimination then, is a vital capacity that enables organisms to respond in different ways to different stimuli.

Equipped with this understanding of sensory discrimination, we can return to the projected image scenario and discover that we need not assume any failures of reasoning at all. All we need to assume is that the depicted black tiles trigger many of the same sensory responses as would actual black tiles seen from the depicted viewpoint. On a perceptual level we would have no difficulty in differentiating between the simulated black and an actual instance of black. And if we were practiced in skills of rational judgement there is no reason why these could not enable us to recognise the differences between simulation and reality. Nonetheless it is important to repeat that nothing hangs on these skills. Nor would any improvement in our skills of judgement make the slightest difference to our susceptibility to discrimination failure, to the seeing—in this case— of white as black.

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.