How can anthropic reasoning change the way scientists compare probable universes?

Anthropic reasoning shifts how scientists compare probable universes by introducing the requirement that a universe must be capable of supporting observers.1

In models that make probabilistic predictions about the structure of the universe, scientists might otherwise erroneously compare the observed universe only to those deemed most probable by a specific theory. Anthropic reasoning corrects this by requiring that the comparison instead be made against universes that are most probable given the condition that they permit the existence and evolution of life.1

This approach is particularly significant in the context of the multiverse and the eternal inflation/string landscape model. In these frameworks, physical constants may take many different values across a vast collection of universes. Because most of the multiverse may be uninhabited, anthropic reasoning allows scientists to explain why observed constants—such as the cosmological constant—take values compatible with life, even if those values appear highly improbable or "unnatural" compared to other possibilities.12

However, this methodology remains debated, as it is difficult to define the precise requirements for life and to compute or compare probabilities across different types of universes.2