The initial conditions of our universe allow for many possible histories. In some, the physics of the cosmos prevents intelligent beings from evolving; in others, beings like us can appear. Our existence proves that our universe is one of a relative few with histories that permit the evolution of life forms. This is called the anthropic principle—named for beings who are anthropic, or human-like—which “says that the universe has to be more or less as we see it, because if it were different there wouldn’t be anyone here to observe it” (55).

Only certain types of universes can be compatible with life, much less scientifically oriented life. For example, a two-dimensional universe wouldn’t permit creatures to evolve since their digestive tracts would split them in two (and life forms might have no way to communicate unless they happened to bump into each other). For this reason, it’s useful to limit consideration of possible universes to those of which the space contains three dimensions and of which other characteristics are compatible with life such as ours. This simplifies the discussion, and it’s a reminder that only anthropic universes contain minds that can ask questions about that universe.