David Lack artificially induced attack behaviour, and it became possible to study this phenomenon more exhaustively and systematically. It is important to recognize that the simplest experiments are really no more than controlled observations. Belief, purpose, desire, hunger: all these concepts are of processes which can never in fact be observable by the student in the experiment, for they all refer to the inner rat and they are all invented to explain the observable phenomena of the outer rat's behaviour. Stereotyped patterns of behaviour are very unlikely to develop if the animal is placed in the experimental situation without having first been deprived of food for a while. Social pressures and example at a human level may be merely one other way of maximising the chances that a particular pattern of behaviour is developed widely but 'accidentally'.