The Schachter and Singer two factor theory of emotion was presented by researchers Stanley Schachter and Jerome E. Singer in 1962. The theory suggest that emotional states contain two components, one physiological and one cognitive. In the case of experiencing a physiological arousal which has no obvious explanation, the individual will attempt to label or describe the state based on the explanations available to him or her, but where an explanation is necessary, not cognizing as to an explanation is needed. The theory also states that physiological states can arise from a cognitive evaluation of an event or situation.
Source: Schachter, S.; Singer, J. (1962). Cognitive, Social, and Physiological Determinants of Emotional State. Psychological Review. 69: 379–399.