What's the trouble with anthropic reasoning?

Selection effects in cosmology are often invoked to "explain" why some of the fundamental constant of Nature, and in particular the cosmological constant, take on the value they do in our Universe. We briefly review this probabilistic "anthropic reasoning" and we argue that different (equally plausible) ways of assigning probabilities to candidate universes lead to totally different anthropic predictions, presenting an explicit example based on the total number of possible observations observers can carry out. We conclude that lacking a fundamental motivation for selecting one weighting scheme over another, anthropic reasoning cannot explain the value of Λ. © 2006 American Institute of Physics.