What appears to be in a certain way, but is
not really so.
Note it is not said one necessarily believes in an
illusion, though of course one may. (See: Delusion.)
There are quite a few common illusions that are and have been widely known
since Antiquity, like appearance of a straight stick when held in clear water (twisted), the
appearance of the moon when it is close to the horizon (large), the appearance of
things for those with jaundice (yellowish) etc . These examples also show that
one may be quite certain that what one sees is an illusion, and
know that it
is not as it appears to one.
Illusions are interesting for psychology, because they provide
data about how the human nervous system works i.e. in this case: How it gets tricked
or confused by illusions.
Illusions are also interesting for philosophy, because so
very much of what
human beings have believed or practised was based on illusions, and because
many philosophers have held that much or all of ordinary human