 Aristotle’s doctrine of the four causes—material, formal, efficient and final—can be regarded as a third early contribution to the philosophy of technology. Aristotle explained this doctrine by referring to technical artifacts such as houses and statues (Physics II.3). The four causes are still very much present in modern discussions related to the metaphysics of artifacts. Discussions of the notion of function, for example, focus on its inherent teleological or ‘final’ character and the difficulties this presents to its use in biology. And the notorious case of the ship of Theseus—see this encyclopedia’s entries on  material constitution,  identity over time,  relative identity,  and  sortals—was  introduced in modern philosophy by Hobbes as showing a conflict between unity of matter and unity of form as principles of individuation. This conflict is seen by many as characteristic of artifacts. David Wiggins (1980: 89) takes it even to be the defining characteristic of artifacts.