The Anthropic Principle

(From Chapter 1 of Anthropic Bias):

The modern era of anthropic reasoning dawned quite recently, with a series of papers by Brandon Carter, another cosmologist. Carter coined the term “anthropic principle” in 1974, clearly intending it to convey some useful guidance about how to reason under observation selection effects. We shall later look at some examples of how he applied his methodological ideas to both physics and biology. While Carter himself evidently knew how to apply his principle to get interesting results, he unfortunately did not manage to explain it well enough to enable all his followers to do the same.

...A total of over thirty anthropic principles have been formulated and many of them have been defined several times over—in nonequivalent ways—by different authors, and sometimes even by the same authors on different occasions. Not surprisingly, the result has been some pretty wild confusion concerning what the whole thing is about. Some reject anthropic reasoning out of hand as representing an obsolete and irrational form of anthropocentrism. Some hold that anthropic inferences rest on elementary mistakes in probability calculus. Some maintain that at least some of the anthropic principles are tautological and therefore indisputable. Tautological principles have been dismissed by some as empty and thus of no interest or ability to do explanatory work. Others have insisted that like some results in mathematics, though analytically true, anthropic principles can nonetheless be interesting and illuminating. Others still purport to derive empirical predictions from these same principles and regard them as testable hypotheses.

For an introduction to the issues surrounding the Principle, and how Nick answers these issues in Anthropic Bias, you may want to look at his Primer on the Anthropic Principle, his Primer on the Doomsday Argument, and his answers to frequently asked questions that these raise.