When no more urgent need is occupying your attention, it pays to extend your knowledge, and hence your competence and your security, by going out and actively exploring. So useful is this that evolution has installed it in many species as one of their basic drives. Rats who are allowed to become thoroughly familiar with a maze will quickly explore a new section that is added to it, even thought they are being consistently and adequately fed elsewhere. Monkeys kept in a box will spend hours fiddling with mechanical puzzles even thought they receive no reward for doing so. Human being who have volunteered to take part in a ‘sensory deprivation’ experiment, in which all they have to do, to earn forty dollars a day, is to stay in a room with no stimulation, rapidly become desperate for something – anything – to feed their minds, and will repeatedly press a button to hear a voice reading out-of-date stock market quotations