Fear is an emotion induced by a threat perceived by living entities, which causes a change in brain and organ function and ultimately a change in behavior, such as running away, hiding or freezing from traumatic events. Fear may occur in response to a specific stimulus happening in the present, or to a future situation, which is perceived as risk to health or life, status, power, security, or in the case of humans wealth or anything held valuable. The fear response arises from the perception of danger leading to confrontation with or escape from/avoiding the threat (also known as the fight-or-flight response), which in extreme cases of fear (horror and terror) can be a freeze response or paralysis. In humans and animals, fear is modulated by the process of cognition and learning. Thus fear is judged as rational or appropriate and irrational or inappropriate. An irrational fear is called a phobia. Psychologists such as John B. Watson, Robert Plutchik, and Paul Ekman have suggested that there is only a small set of basic or innate emotions and that fear is one of them. This hypothesized set includes such emotions as joy, sadness, fright, dread, horror, panic, anxiety, acute stress reaction and anger. Fear should be distinguished from, but is closely related to, the emotion “anxiety”, which occurs as the result of threats that are perceived to be uncontrollable or unavoidable. The fear response serves survival by generating appropriate behavioral responses, so it has been preserved throughout evolution.