The Anthropic Principle

by

Alasdair Richmond

• LAST REVIEWED: 26 August 2011
• LAST MODIFIED: 26 August 2011
• DOI: 10.1093/obo/9780195396577-0135

Introduction

The enduringly controversial “anthropic principle” was baptized by Brandon Carter in 1974. Seeking a balance between excessive anthropocentrism and excessive insistence on human typicality, Carter sought to summarize the complex interrelations between our existence as observers and the physical conditions we observe. All anthropic arguments note that conditions needed to produce context-sensitive observers set restrictions on the conditions such observers will probably observe. As Carter stresses, anthropic reasoning applies to observers tout court and not exclusively to human beings. (If you are a silicon-based observer, expect to live in conditions conducive to silicon-based life.) Likewise, anthropic reasoning need not carry teleological or design overtones. The most widely accepted anthropic principle is the weak anthropic principle (WAP): “What we can expect to observe must be restricted by the conditions necessary for our presence as observers” (Carter 1974, p. 291, cited under General Overviews). Thus, observers who require a delicate range of conditions will almost certainly be found only where those conditions are met. Context-sensitive observers will likely find themselves observing areas of space-time, which may well be atypical of the universe at large. Hence, WAP suggests we should beware of extrapolating the conditions we observe in our neighborhood to the universe as a whole. Thus, some claim anthropic reasoning offsets the Copernican “principle of mediocrity,” which counsels us to view ourselves as being as typical as possible. However, any tension between anthropic and Copernican thinking may be more apparent than real (see, e.g., Bostrom 2002, cited under General Overviews, or Roush 2003, cited under Ancestors of Anthropic Reasoning). The strong anthropic principle (SAP) generalizes WAP and says the presence of context-sensitive observers suggests the universe must be amenable to the evolution of such observers: “The Universe must have those properties which allow life to develop within it at some stage in its history” (Barrow and Tipler 1986, p. 21, cited under General Overviews). Further extensions of anthropic reasoning include the participatory anthropic principle (PAP) and the final anthropic principle (FAP). PAP generally specifies that observers in some sense determine or create the physical properties they observe, whereas FAP states that life, once created, will (or must) endure for all future time. This article offers brief guidance on overviews and anthologies, anthropic design arguments, testing anthropic arguments, and the notorious anthropic-related doomsday and simulation arguments. The primary focus is on philosophical anthropic works. Hence, little attention is paid to teleology in physics or biology, still less to quantum measurement problems or natural selection. Some less well-known pieces have been chosen partly in hopes they become better known.

General Overviews

The classic presentation of anthropic arguments is the comprehensive Barrow and Tipler 1986, which surveys a host of scientific phenomena amenable to anthropic explanation, outlines the philosophical ancestors of anthropic reasoning, and develops a highly controversial series of arguments designed to establish that human life is destined to play a unique role in the future evolution and maintenance of the universe. However, in many respects Barrow and Tipler 1986 offers a somewhat idiosyncratic view of anthropic thinking in that the authors offer several arguments designed to suggest that human beings in particular (not merely observers in general) may play a significant role in the sustenance and future of the universe. Such anthropocentric applications of anthropic thinking are not in line with the initial formulation in Carter 1974 or with many others. Likewise, Barrow and Tipler 1986 often uses “anthropic” as a synonym for “teleological,” admittedly a usage that friends and foes of anthropic thinking have often been happy to follow but that is nonetheless misleading. Bostrom 2002 is a useful source of correctives and alternative viewpoints to those expressed in Barrow and Tipler 1986. In particular, Bostrom 2002 argues that the more teleologically inspired versions of anthropic reasoning (especially the final anthropic principle [FAP]) advanced in Barrow and Tipler 1986 are actually antithetical to the original conception of anthropic reasoning in Carter 1974. Barrow 2001 relates anthropic ideas principally to three key issues: the likelihood that humans are just one of many intelligent life-forms in the universe, the explanatory merits (or otherwise) of many-worlds cosmologies, and the strengths of teleological inferences in the physical sciences. Barrow 2001 can usefully be read as updating and supplementing the very well-focused treatment of the “many-worlds” issues in Leslie 1986. Leslie 1989 considers the competing merits of many-worlds hypotheses at length, primarily defending the thesis that cosmic fine-tuning obliges a choice between either many-worlds or design. Smith 1994 challenges these claims in detail. Tipler 1988 is an essential, if idiosyncratic, compilation of important anthropic arguments and applications.

• Barrow, John D. “Cosmology, Life, and the Anthropic Principle.” Annals of the New York Academy of Sciences 950 (2001): 139–153.

  DOI: 10.1111/j.1749-6632.2001.tb02133.x

  Interesting treatment of anthropic considerations concerning the age of the universe and the presence of observers. Considers what anthropic reasoning suggests about the possibility of extraterrestrial observers and other universes and discusses the competing merits of teleological and nonteleological explanations of fine-tuning.

• Barrow, John D., and Frank J. Tipler. The Anthropic Cosmological Principle. Oxford: Oxford University Press, 1986.

  The widest-ranging single volume on anthropic phenomena yet produced. Discusses a very wide range of physical phenomena that may be amenable to anthropic explanation. Major source especially for participatory anthropic principle (PAP) and FAP.

• Bostrom, Nick. Anthropic Bias: Observation Selection Effects in Science and Philosophy. London: Routledge, 2002.

  Far and away the philosophically best-informed treatment of anthropic issues in the literature. Among other noteworthy features, chapter 3 develops a very clear and insightful taxonomy of forms of anthropic reasoning and subjects some of the more speculative forms of anthropic reasoning to incisive criticism. Essential reading in this field. Now available free in its entirety online.

• Carter, Brandon. “Large Number Coincidences and the Anthropic Principle in Cosmology.” In Confrontation of Cosmological Theories with Observational Data. Edited by M. S. Longair, 291–298. Dordrecht, The Netherlands: Reidel, 1974.

  The paper that essentially coined the term “anthropic principle.” Beginning with the large-number coincidences observed in force ratios and so on, Carter argues for the pervasive influence of observer-selection effects in many disparate areas of science. Reprinted in Leslie 1999 (cited under Anthologies).

• Leslie, John. “Anthropic Explanations in Cosmology.” PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association 1 (1986): 87–95.

  Useful overview of the need for, and differing implications of, observer-selection explanations. Develops Leslie’s objections to the Hacking 1987 (cited under The Inverse Gambler’s Fallacy) “inverse gambler’s fallacy” (as of 1986 circulated only in draft manuscript form). Available online for purchase or by subscription.

• Leslie, John. Universes. London, Routledge, 1989.

  Wide-ranging discussion of worlds making, which pursues anthropically relevant topics into more theological territory than many treatments. An excellent discussion in its own right, this volume’s main interest lies in arguing first that cosmic fine-tuning invites explanation, and second that viable explanations are restricted to versions of intelligent design or multiverse hypotheses. Also usefully points out that design and many-worlds hypotheses, although often presented as antithetical, can be upheld together.

• Smith, Quentin. “Anthropic Explanations in Cosmology.” Australasian Journal of Philosophy 72.3 (1994): 371–382.

  DOI: 10.1080/00048409412346161

  Besides offering a clearly argued and scientifically literate critical overview of anthropic explanations generally, develops a case (contra Leslie 1989 and elsewhere) to the effect that observed cosmological coincidences do not in fact invite the postulation of many worlds. Available online for purchase or by subscription.

• Tipler, Frank J. “The Anthropic Principle: A Primer for Philosophers.” PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association 2 (1988): 27–48.

  Useful but somewhat anthropocentric sketch of the various kinds of anthropic principles, offering definitions that explicitly restrict the principle to human or carbon-based life. Available online for purchase or by subscription.