Topics Perception

Question It is generally agreed that perception involves a real object transferring information about itself into the brain of the perceiver, via the sense organs and nerves; and the distinguishing features of this are that the real object is external to the perceiver and public, while the image of it in the brain is internal and private. My question is: illusions are unreal, but they are external and public --- as with the railroad lines meeting in the distance, or the Sun and the Moon being the same size during an eclipse. So are illusions real, or unreal?

Accepted:
March 15, 2006

Comments

Illusions are "real" in one sense and not in another.

Unreal: The railroad lines look to me as if they meet in the distance, but they don't. In this case, reality isn't as it appears to me.

Real: I am part of reality, and so are my mental states. One of those is that the railroad tracks (falsely) appear to me to meet in the distance. This is a robust illusion that I and many others are really subject to.

There are really two different kinds of "illusions" one might have in mind, and they are "real" in different senses.

Consider first the railroad tracks. We can describe this phenomenon in purely geometric terms. Take a point P and a line segment AB. Then as AB is moved further away from P the angle <APB becomes smaller, and as the distance tends to infinity, the angle <APB tends to zero. So what we're dealing with is just a fact of geometry.

On the other hand, consider the horizon illusion: When the moon is close to the horizon, it looks much larger than when it is overhead. My understanding is that it still isn't well understood why, but whatever the reason, it has something to do with how our visual system works. So this illusion is real not in a geometrical sense but in a broadly psychological one.