actus

Actus is a Latin noun meaning act, action, or deed. In English-language discourse, it is used as a technical term to denote an actualized state or performance, as opposed to potentiality. The word appears in various fields, often in Latin phrases preserved in scholarly writing, such as actus reus or actus purus.

In Aristotelian metaphysics, actus (actuality) is the realized state of a thing, contrasted with potency (potentia), which is the capacity to become otherwise. Aristotle distinguishes between what a thing is in actuality and what it could become. Later Scholastics expanded these ideas, with discussions of actus and potentia shaping theories of essences, change, and being. The concept of God as actus purus—an entity with no potential to change—emerged in medieval theology and has influenced subsequent philosophical and theological thought.

In law, actus reus translates as the “guilty act” and denotes the physical or external component of a crime that the prosecution must prove. It is typically contrasted with mens rea, the mental state or intent accompanying the act. The precise content of an actus reus varies by jurisdiction and offense, but the distinction between conduct and culpable mindset remains central to criminal liability.

Beyond philosophy and law, actus appears in theological and philosophical discussions as part of Latin vocabulary in academic writing and in discussions of action, agency, and moral responsibility. The term helps distinguish actual performed activity from potential or imagined states, a distinction that recurs across disciplines when analyzing change, causation, and responsibility.