A cloud can look like a camel, but a camel is unlikely to look like a cloud. This is so because the signifier must be able to stand for the whole category of the signified. The cloud looks like all camels, but no camel looks like all clouds.
Rudolf Arnheim (1990). “Parables of Sun Light: Observations on Psychology, the Arts, and the Rest”, p.245, Univ of California Press
