  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 in absence
of a fundamental motivation for selecting one weighting scheme over another the
anthropic principle cannot be used to explain the value of Lambda. .
