Suppose, said Eysenck, there were a demon ‘sitting near the point where the long pathways of the central nervous system enter into the lower pathways of the brain’. The demon has two levers, one marked ‘excitation’, the other marked ‘inhibition’. ‘Whenever sensory stimuli are coming in through these pathways, he presses sometimes one lever, sometimes the other, sometimes both. Stimuli produced by the levers are then sent on to the brain, where they either facilitate the passage and the interplay of the incoming neural stimuli, or suppress and inhibit them. In part, therefore, the demon acts as a kind of amplifying valve, and part as a suppresser’ (Eysenck, 1965a). And suppose that some demons are right-handed and use the inhibition lever more, while others are left-handed and use the excitation lever more, while the rest use both levers equally frequently.

Eysenck is offering a model of human temperament. His demon's preference for excitation or inhibition lever can:

• help determine how sociable and impulsive a person is,

• shape a person's political opinions,

• turn him/her into a criminal or a good radar operator,

• make his/her sex life more active and varied.

Temperament theories postulate a bodily base to personality. They are very ancient; the Greek physician Galen, writing in the second century AD, distinguished melancholic, choleric, sanguine and phlegmatic temperaments, which depend on the balance of four humours, or bodily fluids. More recently, Sheldon (1942) argued that physique shapes personality; he distinguished three body types: endomorphic or fat, ectomorphic or tall and thin, and mesomorphic or muscular, which tended to go with jolly, miserable and aggressive personalities respectively. Allport (1937) defined temperament as ‘the characteristic phenomenon of an individual's emotional nature, including his susceptibility to emotional stimulation, his customary strength and speed of response, the quality of his prevailing mood, and all peculiarities of fluctuation and intensity of mood’. Most modern temperament theories, including Eysenck's, look to the brain and nervous system for a physical basis for personality.