Illusions are among our daily 
perceptions, but a more general 
answer to the question is the 
one given in the first chapter: 
Both illusions and veridical 
percepts must be 
manifestations of the same 
lawful processes of perceptual 
functioning. Therefore, we 
stand to gain as much 
knowledge by studying the one 
as by studying the other, and 
ultimately our theories must be 
able to explain both. Research 
on geometrical illusions has 
been active for over a century, 
but—–and this is an 
embarrassment to investigators 
in perception—–they remain, 
for the most part, unexplained. 
Of course, progress has been 
made and theories have been 
advanced, but, as we shall see, 
no one theory does justice to 
them all.

    Rather than attempt to 
catalogue all the known 
illusions, I will restrict the 
discussion to just a few of them. 
They are among the better 
known and can serve the 
purpose of illustrating the 
major theories.