Definition
The anthropic principle is the observation that the universe we observe must have properties compatible with the existence of observers — because if it did not, there would be no one to observe it. The constants and conditions of the cosmos, from the strength of gravity to the value of the cosmological constant, fall within narrow ranges that permit stars, chemistry, and life. The anthropic principle is the formal recognition that this is, in a sense, unsurprising: any universe containing observers must be observer-permitting.
The principle is often invoked in the fine-tuning debate, where the apparent precision of these constants is taken by some as evidence of a designer.
Why it matters
How it works
The reasoning runs as follows. If there are many universes, or many regions, with different constants, then observers will necessarily arise only in the rare ones tuned for life — and will naturally find their own surroundings hospitable. The fine-tuning is then a selection effect rather than a sign of intent, much as the apparent fit of organisms to their environments is explained by selection rather than design.
Dawkins treats this as the cosmological counterpart to the Darwinian reply to the design argument. Defenders of a fine-tuning argument respond that the anthropic principle, even granting a multiverse, leaves open why any life-permitting reality exists, and that positing many universes is itself a substantial assumption. The interpretation of fine-tuning remains genuinely open in physics and philosophy.