Etiology (/iːtiˈɒlədʒi/; alternatively aetiology or ætiology) is the study of causation, or origination. The word is derived from the Greek αἰτιολογία, aitiologia, "giving a reason for" (αἰτία, aitia, "cause"; and -λογία, -logia). The word is most commonly used in medical and philosophical theories, where it is used to refer to the study of why things occur, or even the reasons behind the way that things act, and is used in philosophy, physics, psychology, government, geography, spatial analysis, medicine, theology, and biology in reference to the causes of various phenomena.