Exposure therapy is based on the principle of respondent conditioning often termed Pavlovian extinction. The exposure therapist identifies the cognitions, emotions and physiological arousal that accompany a fear-inducing stimulus and then tries to break the pattern of escape that maintains the fear. This is done by exposing the patient to progressively stronger fear-inducing stimuli. Fear is minimized at each of a series of steadily escalating steps or challenges (a hierarchy), which can be explicit ("static") or implicit ("dynamic" — see Method of Factors) until the fear is finally gone. The patient is able to terminate the procedure at any time.