Friday, 25 March 2011

Are Perceptual illusions always necessarily illusions?

I'm sure that all of us are astounded that the squares A and B are actually the same colour. It is the shadow cast over B by the cylinder which makes us think otherwise. What this suggests is a quite incredible illusion.

However I think there is a pervasive naivety about the nature of perception. Most of us doubtless feel that we see the external world directly. But we emphatically do not.

Consider a red rose. We think of a red rose as being the same colour throughout the day. However the light from the Sun reaching the Earth varies throughout the day. When the Sun is low in the sky, lots of blue light gets scattered away since the sunlight has to travel through a greater quantity of air. So if we were to passively see colours "as they really are", then the colour of our red rose would change throughout the day. Indeed the colour of all objects would change throughout the day. But in fact our rose seems to stay pretty much the same colour throughout the daylight hours. Why is this?

The answer lies in the fact that we do not in fact simply passively see what is out there. Rather the brain performs certain operations on the data coming through our senses and presents it to our consciousness in a form that we can make sense of. Everything we ever see is in fact a hypothesis about how the world is. Thus we have an implicit theory about the external world that it contains objects which have specific intrinsic colours. Hence the brain will perform those operations which ensure that objects do indeed appear to be the same colour throughout daylight hours.

This applies not just to colours, but everything we perceive through our 5 senses. In a way then everything we ever perceive is an illusion. But I think this is misleading.

Let's consider the "illusion" above again. If this were a real 3D object and we were to approach it and view it from various angles, then we would see that squares A and B are very different colours. Indeed their intrinsic colours would be precisely as we perceive them in the illusion above.

But in that case what justifies us in labelling it as an illusion? If this were a real object that we are seeing, then squares A and B are very different colours. Our senses are not deceiving us. Indeed if someone claimed to see the squares as being precisely the same colour, then it is doubtful that he could proficiently visually apprehend his environment.

This is not to say we never perceive illusions. Sometimes we seem to see something, but which on closer inspection turns out to be something else entirely. Or sometimes what we seem to visually see is not consistent with our other senses.


  1. Read the Medieval Guide first - only because it's further back in history, and it's the first one he wrote. But they're both good.



    1. OK thanks Mary! Wondered what the heck you were talking about at first!

  2. Yes, the idea that something is "really this or that colour" is nonsense. It may have such-and-such a pigment combination if we perform some type of analysis, but the colour it "is" - is the colour we experience, surely?

    But we are usually ignorant of how we come to be in a world of things which persist, rather than shapes which continually mutate as we move about.

    Sometimes I think we can catch the process in action. Usually, it's as if we have a vast 'perceptual space' in which objects are experienced. There are no objects out there. Rather, objects are concepts are assembled and learned and then experienced. Sometimes, though, we can have the 'raw experience' before it snaps into object form.

    For instance, looking at a painting and just seeing circles and lines - then suddenly we see: it's a portrait of a person in an office. In real life, this tends to arise from perspective confusion - say, a reflection off part of a surface seeming to be separate and nearer than the rest of the shape, then suddenly we realise what we are seeing and it 'snaps'.

    Ropes and snakes!

    If we spend a bit of time, we can actually directly experience that we are having an indirect sort of "dream inspired by the [assumed] senses". For instance, we can do experiments to direct our attention to "where we are looking out of" and we find there is a gap, and this leads us to experience the background space of our awareness. We can actually experience ourselves living in a George Berkeley landscape.

    The only problem is, this means we have to accept that if there is an "underlying world" then it is inaccessible to us, and therefore doesn't exist except as a principle of faith that there is a "coherence" to experience. For instance, if we hold our hand up in front of us, that isn't our "real hand", it's just an image in awareness. The root of the experience, our "real hand" (if we assume such exists), isn't even "over there" it's somewhere else parallel and completely out-with the space we are experiencing.

    So, does this mean that our daily life is totally an illusion - as in, not as we think it is, in a solid world?