To exist potentially is to be in a state from which a certain actuality can be reached under the right conditions. To exist actually is to have arrived at that actuality — to be fully what one is or what one can do. A builder is actually a builder when building; between jobs, the capacity remains real but unexercised. Aristotle insists that both modes of being are genuine: potentiality is not mere absence but a real disposition in the thing, waiting to be realised.
Although potentiality comes first in the order of time — a seed before a tree, an embryo before an adult — actuality has priority in every other sense. It is prior in definition: to explain what a potential thing is, we must refer to the actuality it aims at. It is prior in substance: the fully developed form is more fully real than the undeveloped matter. And it is prior in time, in the ultimate sense: the actuality that produces this potentiality must itself have existed first. The fully actual always precedes the merely potential in the chain of causation.
The distinction allows Aristotle to give a definition of change (kinēsis) without paradox: change is the actuality of what exists potentially, insofar as it is potential. A stone warming in the sun is moving from the potentiality of being warm to the actuality of being warm — the change just is this passage in progress. This definition resolves the Eleatic puzzle about change (how can what is not become what is?) by showing that what changes is neither fully actual nor fully absent but genuinely in between: potential and in the process of becoming actual.
Actuality and potentiality are introduced in Metaphysics IX.1 and developed throughout the book. The distinction is also deployed in the Physics (to define motion) and in De Anima (where the soul is the actuality of the body). It is one of Aristotle's most far-reaching conceptual innovations.
