Dollard et al. (1939) proposed that aggression was due to frustration, which was described as an unpleasant emotion resulting from any interference with achieving a rewarding goal. Berkowitz extended this frustration–aggression hypothesis and proposed that it is not so much the frustration as the unpleasant emotion that evokes aggressive tendencies, and that all aversive events produce negative affect and thereby aggressive tendencies, as well as fear tendencies. Besides conditioned stimuli, Archer categorized aggression-evoking (as well as fear-evoking) stimuli into three groups; namely, pain, novelty, and frustration, although he also described "looming," which refers to an object rapidly moving towards the visual sensors of a subject, and can be categorized as "intensity."