There are many different formulations of the anthropic principle.
Anthropic reasoning is often used to deal with the notion that the universe seems to be finely tuned for the existence of life. Proponents of the anthropic principle argue that it explains why this universe has the age and the fundamental physical constants necessary to accommodate conscious life, since if either had been different, we would not have been around to make observations. The anthropic principle, also known as the 'observation selection effect', is the hypothesis, first proposed in 1957 by Robert Dicke, that the range of possible observations that we could make about the universe is limited by the fact that observations could only happen in a universe capable of developing intelligent life in the first place. For the book by Nick Bostrom, see Anthropic Bias (book).
