After all, the illusion here is 
only an illusion in a picture. 
Although viewing pictures may 
be an important part of 
perception in everyday life, 
perception of the world is far 
more important. In this scene, 
the extents that look different 
in length would be different in 
length in the world. We would 
not describe that perception as 
illusory, even though the 
lengths between the images of 
lines on the retina is equal. 
Illusions must be defined as a 
discrepancy between what is 
perceived and what is 
objectively present in the 
world, not as a discrepancy 
between what is in the retinal 
image and what is present in 
the world. However, as we have 
already seen and will see in 
subsequent chapters, 
perception in daily life is shot 
through with illusions, 
including those of the 
geometrical kind to be 
addressed in this chapter. 
Sometimes we are aware of 
them, but more often we are 
not--for the simple reason that