Aristotle's views on abstraction explain how he can hold that parts of animals are both secondary substances and relata . The wing of a bird, qua being a part, is a relatum ; that wing, per se, is a secondary substance. Aristotle's views on motion and change make clear his position on parts of animals. Motion and change are relations. Strictly speaking then, motion and change hold between relata . Such relata are accidents of subjects: their ultimate subjects are the substances; yet their proximate subjects may be its accidents taken as subjects in their own right, like surfaces and numbers. When the subjects themselves are in the category of relation , the change is instantaneous; otherwise it can be gradual. As parts of animals can undergo gradual change, they are not relata ; rather they are substances. Still these substances qua parts are relata . For a part of a woman like a hand to be a part of her, it must function as a hand. When it does not function it is a hand in name only. A non-functioning hand is still a substance with a history of its own but is not a fully actualized part of the substance the human being. Aristotle also allows for parts that are not actual but potential in various degrees.