Teleology is the study of purpose or design in nature. It asks whether natural processes have ends or goals. Aristotle argued that everything has a telos, a purpose or final cause. An acorn's telos is to become an oak tree. A knife's telos is to cut. The telos explains why something is the way it is. In the Middle Ages, teleology was central to theology. God designed the universe with a purpose. In the modern period, teleology fell out of favor in science. Darwin's theory of evolution explained biological adaptation without appealing to purpose. Natural selection is not goal-directed. It is a mechanical process. But teleology has not disappeared. Biologists still speak of function. Philosophers debate whether functions are teleological or causal. Some argue that teleology is necessary to understand living systems. Others argue that it is a useful fiction.

Teleology is also used in ethics. Utilitarians evaluate actions by their consequences, which is a form of teleological ethics. Virtue ethicists focus on the telos of human life, which Aristotle called eudaimonia. Teleology raises questions about meaning and purpose. Is there a purpose to the universe? Is there a purpose to human life? The answers are not given by science alone.

Types of teleology

• Natural. Purposes in nature, as in Aristotle's biology.
• Divine. Purposes designed by God.
• Ethical. Evaluating actions by their ends.
• Functional. Explaining biological traits by their function.

Teleology is not the same as determinism. Determinism says events are caused by prior conditions. Teleology says events are directed toward ends.