The Checker-Shadow "illusion" Part 2.

Around 15-20 years ago I had this sudden insight, a sudden understanding. It was pertaining to so-called perceptual illusions.  I suddenly realised that many so-called perceptual illusions are not in fact illusions at all. Close to ten years ago I wrote a blog post regarding the checker-shadow illusion where I explained that, in my opinion, properly speaking, this is not in fact an illusion at all. Before reading further I recommend that people read that blog post. I would also recommend that people read another blog post by me on perceptual illusions here.   And, although not necessary to read, this blog post too might be of interest.  

On searching on the net I've only ever been able to find one person who holds the same opinion as I do that this shadow-checker "illusion" is not in fact an illusion at all.  And that was merely on a discussion forum.  Indeed, I have been vehemently attacked on this issue by many people (although it is clear to me they simply completely fail to understand my argument). Anyway, just a couple of days ago, I discovered a very interesting review of a book regarding colours and which talks about the checker-shadow illusion.  It says:

“Illusion” implies that our system is fooled, but as far as useful information goes, the checkerboard interpretation is probably better. Try as they might, mathematicians can’t make the computers see the checkerboard. Rather than a demonstration of how easily fooled we are, optical illusions like this one are examples of the brain’s mysterious and irreplicable abilities. It interprets its environment with a sophistication that exceeds our ability to measure and reconstruct physical phenomena. The usual framing has it wrong: Despite A and B having the same SSR, humans are still able to see the checkerboard.

First of all, it's gratifying to at last find someone else that agrees with me.  Two other people actually, both the author of the book and the reviewer.

But, anyway, this underscores the fact that in certain respects computers, at least if merely using visual information, cannot see as well as we do.  The fact that computers cannot see the checkerboard implies that they cannot see their environment very well at all.  This is why autonomous cars can't just rely on cameras but require other sensing methods such as LIDAR.  It also partially explains why fully autonomous cars might still be decades away.  Back in 2014 I predicted 2060, which at that time was vastly later than all the predictions of the various pundits.  Almost to the man they thought that fully autonomous cars would be widespread within 5-20 years.

So, despite using highly sophisticated algorithms, computers still cannot see as well as we do.  An interesting question is why can't they?  Why can't they see the checkerboard?  If it is merely the brain that enables us to see, then whatever physical processes are involved, they surely should be able to be matched by suitably sophisticated algorithms.  To me their failure in this regard suggests that something else is required, that consciousness in and of itself  is playing a pivotal role that is enabling us to see proficiently.

The Tabletop Illusion

I'm currently reading the soul fallacy by Julien Musolino. In it he tries to stress that we are constantly being fooled by our subjective first person perspective and in our judgements. That we need the objective third person perspective as provided by science to tell us how reality really is.

He tries to illustrate his point by the following perceptual illusion of two tables (the Roger Shepard tabletop illusion).

I think we would all agree that the two tables are different shapes. But, when we rotate the left table 90 degrees clockwise and move it over the other table, we find they are both exactly the same shape! Moreover, even when we are made aware of this fact we cannot help but see that the tables are very different shapes. Julien Musolino finds this very significant. In the the soul fallacy he says:

[This illusion] pits our first-person perspective—what our senses subjectively reveal to us—against the third-person perspective—what the result of objective measurement demonstrates. The story of the demise of the soul, to a large extent, reflects the triumph of the third-person perspective over its subjective, first-person counterpart. But there is more to Shepard's illusion than meets the eye. The analogy contains two additional virtues. After this brief demonstration, I do not know anyone who would continue to insist that one tabletop really is longer than the other because of the way they look ... The analogy's second virtue is that it reveals to us how stubborn first-person impressions can be in the face of objective evidence.

I think this is flawed reasoning. Indeed, it seems to me to be inappropriate to call this an illusion at all.

First of all, we need to draw a distinction between the image of the tables, and the actual tables themselves that they represent. The above picture is an image of two tables.

Having put that aside, let's suppose there are two tables in front of us that are in our visual field. When looking at the tables, let's also suppose that the image of the tables on our retinas closely approximates to the image of the tables we see above on our computer screens. Just as the shapes of the images of the two tables is the same on our computer screens above, so too will the shapes of the two images of the tables on our retinas be the same.

Are we fooled in the latter case? I would say not since if we approached the two tables, look at them from different angles, run our fingers over them and so on, we would definitively establish the two tables are of differing shapes. Likewise, if we are to take the drawing of the two tables above as actually depicting tables rather than arbitrary lines representing nothing, it is not fair to say we are being fooled here either. In other words, there is no illusion, as such.


We need to understand how are senses work. We don't simply passively perceive what's out there. The data we receive through our senses is hopelessly inadequate for us to see the world as it truly is in and of itself. The actual ability to see is supplied by our implicit expectations gained from our previous acquaintance with the world. That is to say, what we actually see is shaped and moulded by all of our previous visual experiences. In effect, we have an implicit unconscious theory about how the world is and this is instrumental in shaping what we actually perceive.

Without such an implicit theory, we wouldn't be able to perceive at all, at least not a 3D world of objects at various distances. We'd just see a splodge of colours in a 2-dimensional plane. In other words, we'd see the world as a computer or robot would; namely as depicted from a third person perspective shorn of any "illusions" that any minds can add.

In short, this "illusion" is not an example of us being fooled. If we didn't experience such "illusions", then we wouldn't be able to see at all! This is why autonomous cars -- which do not have the benefit of illusions to apprehend the environment correctly -- shouldn't rely upon cameras alone. They need other instruments to effectively detect the environment, such as LIDAR. Even then, they are not as proficient as human drivers in urban environments. My personal expectations are that fully (level 5) autonomous vehicles will not be widespread for a few decades yet (back in 2014 the date I gave was 2060).


So, is the subjective first person perspective inferior to the objective third person perspective as Julien Musolino claims? Certainly not when it comes to visually apprehending the environment. So much for, as he puts it, "the triumph of the third-person perspective over its subjective, first-person counterpart". This is not to deny that we are often fooled and that the third person perspective, as provided by science, is the more accurate. But I don't think this has any relevance for deciding whether, on the one hand, our essence is a soul or, on the other hand, a sophisticated biological machine. But I'll address that issue when I come to reviewing his book.

Two other relevant similar blog posts by me are:

Are Perceptual illusions always necessarily illusions?  (essentially the same argument I make in this blog post, but I wrote it over 9 years ago).

Perceptual illusions show our minds construct reality

Perceptual illusions show our minds construct reality

via GIPHY


This is Richard Wiseman. He's a skeptic of the paranormal. I think he's especially fond of perceptual illusions as they show how easily people are fooled. The message he's essentially trying to convey is: 'you think you've seen something weird? Look at how easily we're fooled!'

Perhaps so.  But I think it also shows that we might sometimes be unable to see the unusual and anomalous in our visual fields.  Allow me to explain.

We don't just passively see what's out there. All the light that enters our eyes simply gives us a flat 2 dimensional plane of various shades of colour. However, we seem to immediately see a 3D world with objects at various distances. How so?

The answer is that it's derived from our familiarity with the world. In our experience of reality, familiar objects have certain sizes and a certain appearance. The mind uses this background experience to quite literally construct what we see out there. So, even though only a 2 dimensional plane of colour is actually immediately given, we seem to see directly a world in 3D -- a world populated with familiar objects at various distances.

To reiterate, this construction crucially depends on what we're used to seeing. Hence, when we introduce an anomalous object such as an unusually large-sized drinking glass, our minds will be wanting to see it as a normal-sized glass. So, they will be a propensity to see the glass closer and smaller than it really is. However, other cues in the environment normally alert us that what we are seeing is an anomalously large version of a familiar object. But here, in this gif, those other cues are absent, so we naturally see it as a normal-sized glass that is closer than it actually is.

But, what's interesting is that if we were to see something unfamiliar to us, something we've never seen before, there will be a propensity to see it as something more familiar. The upshot of all this is that there might be more unusual and anomalous phenomena out there than we suppose, but we just can't see it.

I suspect it might also have implications for what we appear to see during something like a near-death experience. If our vision in an afterlife realm works anything like normal vision, then our minds with their implicit expectations will shape and mould what we actually perceive.

A similar post by me written a few years ago:  Are Perceptual illusions always necessarily illusions?