The occurrence of an experience in itself indistinguishable from a perception of something, but without an appropriate external cause. Hallucination is sometimes distinguished from pseudo-hallucination, in which the experience occurs but is not mistaken for the perception of an external object. Both are distinct from illusion, in which there is an external source, but its nature is mistaken (a mirage is thus an illusion, but not a hallucination). The possibility of hallucination is a frequent starting-point for the distinction between appearance and reality. See also illusion, arguments from.