Topics in Philosophy of Mind
Sights: Some Perceptual Illusions

The vertical red lines over this photo are exactly the same size. Why do some look longer? The Empiricist explanation is that you 'read' them in terms of the background photo. If the red lines were actually poles in the scene, the ones that are positioned farther away would have to be taller, and so we see them that way.

Which red line looks longer? The Empiricsit explanation is that you see the vertical lines as something like railroad tracks, receding away from you. If they were, then something that appeared the way the top horizontal bar does would have to be longer. You infer that it is longer and therefore see it as longer.

How many bumps do you see on the left? How many dips?
How many bumps do you see on the right? How many dips?

In fact, the right image is just an inverted form of the left one. We see bumps when the lighter form of the circle is on top, and the darker is on bottom. We see dips when the darker part is on the top and the lighter is on the bottom.


When a light source is overhead, bumps are shaded lighter on top. When the light source is overhead, dips are shaded lighter on the bottom. Our perceptual systems therefore seem to be working on the assumption that the light source is overhead. If you stood on your head, you would find that the bumps and dips seemed to reverse--what was a bump when you are standing upright becomes a dip when you look at the image standing on your head. That seems to indicate that our perceptual system not only works with an assumption that the light source is above the earth, for example, but that comes from above our heads no matter where our heads happen to be.

Some Auditory Illusions

The Shepherd tones

When you press the button, you will hear a series of tones. Heard one by one, each tone seems to be higher than the one before. But they can't just keep going up, and they still sound like they are somewhere in the middle range.

The Auditory Rabbit

This is a tricky and subtle illusion, which requires balanced stereo speakers or headphones.
  • When you press the 'left' button, you will hear a series of clicks to the left.
  • When you press the 'right' button, you will hear a series of clicks to the right.
  left half only

  right half only

When you press the third button, you will hear a series of quick clicks that seem to move smoothly from left to right. Some people describe it as sounding like a thumb being drawn across the teeth of a comb. That is the auditory rabbit, hopping from left to right.

In fact, however, this is just the clicks from the left followed immediately by the clicks to the right. The Empiricist would say that the motion between them is something you have inferred.