Big ideas come from the unconscious. This is true in art, in science, and in advertising. But your unconscious has to be well informed, or your idea will be irrelevant. Stuff your conscious mind with information, then unhook your rational thought process. You can help this process by going for a long walk, or taking a hot bath, or drinking half a pint of claret. Suddenly, if the telephone line from your unconscious is open, a big idea wells up within you.
