ABSTRACT
This chapter shows how the experimental psychologist, with behavioural observations and measurements, with mathematics and the computer, can take the total behaviour apart into meaningful and useful abstractions, such as traits of general personality, ability, or motivation. It also shows how modem quantitative psychology can represent the unique individual personality and the equally unique situation, and combines them in a 'specification equation' to predict the response which the person is likely to make. The appropriate procedure in science is to choose the simplest model that seems workable and stick to it unless and until it fails in some respect. The additive and linear model evidently suffices for a great deal of human behaviour, and it can be modified at points like the above where some particular trait behaves in an unusual manner. Behavioural traits are the necessary negotiations or compromises between the physiological demands of the organism, and the physical and social demands of the environment.
