Visual appearance and illusion

JimJim asked:

Visual size is illusory: it shrinks in all three dimensions. Before we correct for this, not only do the railroad tracks meet in the distance, but a train travelling down them gets shorter, narrower, and smaller. So my question is: how far away must a visible object be for us to see it real size?

Answer by Geoffrey Klempner

The premise of your question is false. There are visual illusions, which require a special setup to work, but in general things appear the size that they are, no larger or smaller.

You can verify this for yourself easily. Pick an object on the far side of the room and walk towards it. Does the object (a framed picture, say) ‘get larger’ as you move towards it. Of course not. Hold out your arm and look at your hand. Now move your index finger towards your eye. At what point does your finger appear bigger than it is? At no point.

The notion, e.g., that a train travelling away from us ‘gets shorter, narrower, and smaller’ is based on a overly simplified model of perception. When you look at the train as it travels into the distance, the image projected upside down onto your retina gets smaller and smaller. But what you see, what you perceive, isn’t that image. You see the train. Moreover, you see it as a train, that is to say, an constructed object of a kind that maintains its size over time. (I’m ignoring the fact that a train gets longer or shorter if you add or subtract carriages.)

There are common objects that get larger and smaller. A balloon, for example. Let’s say we are watching a clown walking the road with a large balloon. The balloon has a puncture, and is visibly shrinking, getting smaller and smaller as we look on. The clown turns towards us and shakes his head, sadly. Being able to tell when things actually get bigger or smaller is a pretty important ability, don’t you think?

In order to explain this, a distinction is sometimes made between what we ‘actually see, with our eyes’, and the perceptual judgements based on what we see. So, in your example of the train, we ‘actually see’ the train get smaller, but this is then corrected by our judgement.

There are special cases where this is true. The moon in the sky doesn’t look that large. But then when you take into account the information that the moon is a quarter of a million miles away, a quick calculation shows that it must be pretty big if we can see it at all at that distance.

Then there are artificially constructed experimental setups where a man walking across a room appears to get smaller because the ‘room’ in question is designed with a false perspective: we see the room as rectangular, but in fact the far wall is twice the size of the near wall.

In each of these cases, judgement is required to correct what we see, or seem to see. But these are necessarily exceptions to a rule: The rule being that our faculty of perception (eyes, optic nerve, brain — not forgetting our capacity to physically manipulate the objects that we see) is ‘designed’ by evolution to produce veridical appearances. We need accurate information coming through the senses on which to base our judgements. That’s how perception works.

The concept of perception applies not only to the five senses but also to things like understanding what a person is saying. We perceive meaning. Sometimes we can be wrong, and often those errors can be corrected by judgement. But judgement needs something to work on on. Language isn’t a cacophony of sound, or squiggles on a screen or on paper that we then have to interpret — although, as in the special case it can be, e.g., if you don’t ‘know the language’ and have to work what the person is saying from a phrase book.

A good question to ask in alleged cases of perceptual illusion is, How would things look otherwise? Discussing ancient beliefs about the cosmos, one of Wittgenstein’s students once remarked about the fact that the sun appears to go round the Earth. ‘And how would it look if the Earth appeared to go round the sun?’ was his reply. — I’ll leave you with that question to think about.

One thought on “Visual appearance and illusion

  1. With all due respect to Prof. Klempner, he is wrong to say that the premise to my question is false. The question concerns visual size, not real size. Visual size diminishes with distance, real size does not. Visually, parallel lines meet in the distance, in reality they do not. So it is meaningful to ask: at what distance is visual size equal to real size?

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