---
title: "Anthropic principle"
chunk: 3/9
source: "https://en.wikipedia.org/wiki/Anthropic_principle"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:30:47.620122+00:00"
instance: "kb-cron"
---

The observed values of all physical and cosmological quantities are not equally probable but they take on values restricted by the requirement that there exist sites where carbon-based life can evolve and by the requirements that the universe be old enough for it to have already done so.
Unlike Carter, they restrict the principle to "carbon-based life" rather than just "observers". A more important difference is that they apply the WAP to the fundamental physical constants, such as the fine-structure constant, the number of spacetime dimensions, and the cosmological constant—topics that fall under Carter's SAP.
According to Barrow and Tipler, the SAP states that "the Universe must have those properties which allow life to develop within it at some stage in its history." While this looks very similar to Carter's SAP, the "must" is an imperative, as shown by the following three possible elaborations of the SAP, each proposed by Barrow and Tipler:

"There exists one possible Universe 'designed' with the goal of generating and sustaining 'observers'."  This can be seen as simply the classic design argument restated in the garb of contemporary cosmology. It implies that the purpose of the universe is to give rise to intelligent life, with the laws of nature and their fundamental physical constants set to ensure that life emerges and evolves.
"Observers are necessary to bring the Universe into being."  Barrow and Tipler believe that this is a valid conclusion from quantum mechanics, as John Archibald Wheeler has suggested, especially via his idea that information is the fundamental reality and his participatory anthropic principle (PAP), which is an interpretation of quantum mechanics associated with the ideas of Eugene Wigner. (See: It from bit)
"An ensemble of other different universes is necessary for the existence of our Universe."  By contrast, Carter merely says that an ensemble of universes is necessary for the SAP to count as an explanation.
Philosophers John Leslie and Nick Bostrom reject the Barrow and Tipler SAP as a fundamental misreading of Carter. For Bostrom, Carter's anthropic principle just warns us to make allowance for "anthropic bias"—that is, the bias created by anthropic selection effects (which Bostrom calls "observation" selection effects)—the necessity for observers to exist in order to get a result. He writes:

Many "anthropic principles" are simply confused. Some, especially those drawing inspiration from Brandon Carter's seminal papers, are sound, but... they are too weak to do any real scientific work. In particular, I argue that existing methodology does not permit any observational consequences to be derived from contemporary cosmological theories, though these theories quite plainly can be and are being tested empirically by astronomers. What is needed to bridge this methodological gap is a more adequate formulation of how observation selection effects are to be taken into account.
Bostrom defines a concept called the "strong self-sampling assumption" (SSSA), the idea that "each observer-moment should reason as if it were randomly selected from the class of all observer-moments in its reference class." Analyzing an observer's experience into a sequence of "observer-moments" like this helps avoid certain paradoxes, but the main ambiguity is the selection of the appropriate "reference class". For Carter's WAP, this might correspond to all real or potential observer-moments in our universe. As for his SAP, this might correspond to all in the multiverse. Bostrom's mathematical development shows that choosing too broad or too narrow a reference class leads to counter-intuitive results, but he is not able to prescribe an ideal choice.
According to Jürgen Schmidhuber, the anthropic principle essentially just says that the conditional probability of finding yourself in a universe compatible with your existence is always one. It does not allow for any additional nontrivial predictions such as "gravity won't change tomorrow". To gain more predictive power, additional assumptions on the prior distribution of alternative universes are necessary.
Playwright and novelist Michael Frayn describes a form of the strong anthropic principle in his 2006 book The Human Touch, which explores what he characterises as "the central oddity of the Universe":

It's this simple paradox. The Universe is very old and very large. Humankind, by comparison, is only a tiny disturbance in one small corner of it--and a very recent one. Yet the Universe is only very large and very old because we are here to say it is... And yet, of course, we all know perfectly well that it is what it is whether we are here or not.