In psychology we use the term the ‘adequate stimulus’ of any sensation to describe the stimulus to which the sense organ is peculiarly adapted to respond. The adequate stimulus of sight is luminiferous waves of ether; of hearing, it is certain waves of air; of smell, it is certain odoriferous particles in gaseous form; of taste, substances able to produce a certain chemical change in contact with the taste buds on the tongue and palate. The same principle applies to the instincts: the normal stimulus of the instinct of curiosity is any strange object; of fear, any dangerous object; of the maternal instinct, any helpless object. Complexes also have their ‘adequate stimuli’, and may be aroused by special objects which in some way are associated with them. For instance, a red flower may produce a sick headache; the banging of a door may send a man into a fit.