Appearances to the mind are of four kinds. Things either are what
they appear to be; or they neither are, nor appear to be; or they
are, and do not appear to be; or they are not, and yet appear to be.
Rightly to aim in all these cases is the wise man's task.
-- Epictetus (c. 60 AD)
-- Discourses, Book i, Chap. xxvii