In philosophy, potentiality and actuality are principles of a dichotomy which Aristotle used to analyze motion, causality, ethics, and physiology in his Physics, Metaphysics, Nicomachean Ethics and De Anima (which is about the human psyche).The concept of potentiality, in this context, generally refers to any "possibility" that a thing can be said to have. Aristotle did not consider all possibilities the same, and emphasized the importance of those that become real of their own accord when conditions are right and nothing stops them. Actuality, in contrast to potentiality, is the motion, change or activity that represents an exercise or fulfillment of a possibility, when a possibility becomes real in the fullest sense.These concepts, in modified forms, remained very important into the middle ages, influencing the development of medieval theology in several ways. Going further into modern times, while the understanding of nature (and, according to some interpretations, deity) implied by the dichotomy lost importance, the terminology has found new uses, developing indirectly from the old. This is most obvious in words like "energy" and "dynamic" (words brought into modern physics by Leibniz) but also in examples such as the biological concept of an "entelechy".