Principle of teleology Aristotle begins the Politics by invoking the concept of nature (see Political Naturalism). In the Physics Aristotle identifies the nature of a thing above all with its end or final cause (Physiscs II.2.194a28–9, 8.199b15–18). The end of a thing is also its function (Eudemian Ethics II.1.1219a8), which is its defining principle (Meteorology IV.12.390a10–11). On Aristotle's view plants and animals are paradigm cases of natural existents, because they have a nature in the sense of an internal causal principle which explains how it comes into being and behaves (Phys. II.1.192b32–3). For example, an acorn has an inherent tendency to grow into an oak tree, so that the tree exists by nature rather than by craft or by chance. The thesis that human beings have a natural function has a fundamental place in the Eudemian Ethics II.1, Nicomachean Ethics I.7, and Politics I.2. The Politics further argues that it is part of the nature of human beings that they are political or adapted for life in the city-state. Thus teleology is crucial for the political naturalism which is at the foundation of Aristotle's political philosophy. (For discussion of teleology see the entry on Aristotle's biology.)