Guthrie assumed that it is a property of organisms that whenever a response occurs, that response is immediately and completely associated with all stimuli present at that instant. Thus, analysis of behavior form this point of view requires the description of the response and the specification of all afferent activity occurring at the same time. The response in classical conditioning may at first be the UR, but over a number of trials the form and the temporal characteristics of the response may change markedly. The stimulus may also vary over trials, since the response-produced stimuli generated by reactions of the organism are an important portion of the total afferent state.