Visual experience is useful because it creates memories of past stimuli that can later serve as a context for perceiving new stimuli. Thus, you can think of experience as a form of context that you carry around with you.

When you read, you use the context of your prior experience with words to process the words you are reading. Context may also occur outside of you, as in the surrounding elements in a visual scene. When you are reading and you encounter an unusual word, you may be able to determine the meaning of the word by its context. Similarly, when looking at the world, you routinely make use of context to interpret stimuli.

Concentric Circles

You perceive the size of an object by comparing it with its surroundings. This is true of everything you see: trees, buildings, people. You would be shocked to find that a person standing next to his house was taller than the house. You would think it was trick photography if you saw it in a picture. In real life, you might think he had built a doll house for his daughter or a tree house (not yet in a tree) für his son. You would not be inclined to think it was his own house, even if it was very elaborate and complete in every detail.

Consider the three sets of concentric circles. In each of the three drawings—A, B, and C—the inner circle is exactly the same size. Yet the inner circle at A looks as if it must be larger than the inner circle at B; the inner circle at B must, it would seem, be larger than the inner circle at C; and of course, the inner circle at C is much smaller than the inner circle at A. Yet you can measure the diameters of the inner circles and see that they are all identical. The outer circles are all that you have to compare for size when imagining how big or small the inner circles are. There is no doubt that the outer circles become progressively larger as you go from A to B to C. These outer circles distort the way you see the inner ones.

Another example of a size-comparison illusion. The inner circle at A seems larger than the one at B. You have probably guessed that they are the same size. I had thought about making the inner circle at B a little bit larger than the one at A, to see if I could fool you into saying that they are the same size. But that would take away from the spirit, as well as the vividness, of the illusion.