The Anthropic Principle is the simple fact that we live in a universe set up to allow our existence. If the universe were any other way, we would not exist, and would hence be unable to make any observations. Since it was introduced by the theoretical physicist Brandon Carter in 1973, the Anthropic Principle has progressively come into vogue among the physics and philosophical communities, providing a simple explanation for some otherwise very perplexing coincidences. For example, why are certain physical constants so oddly fine-tuned? While some people view this as evidence for a supernatural creator, materialists simply observe that if it were any other way, we wouldn't be here.
Due to the Many Worlds Interpretation of quantum mechanics, many physicists have come to view our universe as one among many--possibly an infinite number, or superset of universes sometimes called "the multiverse." Though we know nothing about the underlying process which could generate such universes, presumably it is random, leading to universes with a variety of different sizes, ages, densities, dimensions, and fundamental physical laws.
Considering a multitude of randomly generated universes, it seems as if many would be hostile to the existence of the delicate, self-perpetuating arrangements of matter known as life. Even a smaller subset would include intelligent observers. If consciousness is only exhibited by a subset of intelligent minds, then there may even exist universes populated by unconscious intelligences, or the hypothetical entities known in philosophy of mind as "zombies." These fascinating lines of thought all follow from the recognition of the far-reaching consequences of the Anthropic Principle.
Anthropic reasoning has been employed in areas ranging from superstring theory--the effort to create a unifying theory of quantum gravity--to predicting the future of the human race; to guessing at the fate of the universe. Use of the Anthropic Principle has been criticized for its almost-eerie ability to contribute to inductive processes in a variety of domains. Also, because it is so new and unusual, critics claim this pricincple has been overextended in certain areas. For example, in The Anthropic Cosmological Principle, John Barrow and Frank Tipler introduce a "Final Anthropic Principle," which states that once intelligent life comes into existence in the universe, it will never die out. Such gung-ho extensions of the Principle have rallied skepticism among certain thinkers. Others feel it is simply too broad to make useful, testable, specific predictions.