It is a study of the processes of creativity and imagination in which Koestler explains that humans are most creative when rational thought is abandoned during dreams and trances. Koestler affirms that all creatures have the capacity for creative activity, frequently suppressed by the automatic routines of thought and behavior that dominate their lives. Koestler's basic idea is that the creative act is a "bisociation" (not mere association) which happens, if two (or more) apparently incompatible frames of thought ("matrices") are brought together by an ingenious mind. In jokes and humour, these conceptual systems are reversed, in the arts and in ritual, they are juxtaposed, in science, they are fused into a new larger synthesis. This corresponds to a "self-assertive" tendency in humour and a "self-transcending" tendency in art, while in science both tendencies are balanced.
Is it illegal to sell laughing gas?
From a biological/evolutionary point of view there've been various theories prosposed but as far as I'm aware there's been nothing conclusively proven.