Irritation, Thus Consciousness

We must assume that consciousness grows from small beginnings; perhaps its first form is a vague feeling of irritation, experienced when the organism has a problem to solve such as getting away from an irritant substance. However this may be consciousness will assume evolutionary significance — and increasing significance — when it begins to anticipate possible ways of reacting: possible trial-and-error movements, and their possible outcomes.
pp. 250- 251

Karl Popper, "Of Clouds And Clocks" in Objective Knowledge