The term Anthropic Principle (AP) was introduced by the physicist Brandon Carter, who stated that "what we can expect to observe must be restricted by the conditions necessary for our presence as observers" (Carter 1974, p. 292). The central idea of AP could be put as follows: We can observe only those states of affairs that are compatible with the existence of observers.

The term has subsequently been applied to all manner of claims, variously obscure and bizarre. This entry restricts its attention to the central and philosophically interesting idea. Carter distinguished what he called the weak version of the principle, according to which "our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers" (Carter 1974, p. 293), and the strong version, which states that "the universe (and hence the fundamental parameters on which it depends) must be such...