universal probability bound

{{Short description|Notion in intelligent design}}{{Intelligent Design}}

A universal probability bound is a probabilistic threshold whose existence is asserted by William A. Dembski and is used by him in his works promoting intelligent design. It is defined as {{quote|A degree of improbability below which a specified event of that probability cannot reasonably be attributed to chance regardless of whatever probabilitistic resources from the known universe are factored in.[http://www.iscid.org/encyclopedia/Universal_Probability_Bound ISCID Encyclopedia of Science and Philosophy] (1999)}}

Dembski asserts that one can effectively estimate a positive value which is a universal probability bound. The existence of such a bound would imply that certain kinds of random events whose probability lies below this value can be assumed not to have occurred in the observable universe, given the resources available in the entire history of the observable universe. Contrapositively, Dembski uses the threshold to argue that the occurrence of certain events cannot be attributed to chance alone. Universal probability bound is then used to argue against evolution.

The idea that events with fantastically small, but positive probabilities, are effectively negligibleNegligible means having probability zero. Effectively negligible means, roughly, that in some operational sense or in some computational sense, the event is indistinguishable from a negligible one. was discussed by the French mathematician Émile Borel primarily in the context of cosmology and statistical mechanics.Émile Borel, Elements of the Theory of Probability (translated by John Freund), Prentice Hall, 1965, Chapter 6. See also [http://www.talkorigins.org/faqs/abioprob/borelfaq.html Citations from Borel's articles]. However, there is no widely accepted scientific basis for claiming that certain positive values are universal cutoff points for effective negligibility of events. Borel, in particular, was careful to point out that negligibility was relative to a model of probability for a specific physical system.Though Dembski credits Borel for the idea, there is clear evidence that Borel, following accepted scientific practice in the foundations of statistics, was not referring to a universal bound, independent of the statistical model used.Cobb, L. (2005) [http://www.aetheling.com/essays/Borel.html Borel's Law and Creationism], Aetheling Consultants.

Dembski appeals to cryptographic practice in support of the concept of the universal probability bound, noting that cryptographers have sometimes compared the security of encryption algorithms against brute force attacks by the likelihood of success of an adversary utilizing computational resources bounded by very large physical constraints. An example of such a constraint might be obtained for example, by assuming that every atom in the observable universe is a computer of a certain type and these computers are running through and testing every possible key. Although universal measures of security are used much less frequently than asymptotic onesFor a precise definition of effective negligibility in cryptography, see Michael Luby, Pseudorandomness and Cryptographic Applications, Princeton Computer Science Series, 1996. and the fact that a keyspace is very large may be less relevant if the cryptographic algorithm used has vulnerabilities which make it susceptible to other kinds of attacks,Though Dembski repeatedly appeals to cryptography in support of the concept of the universal probability bound, in practice cryptographers hardly use measures which are in any way related to it. A more useful concept is that of work factor. See p. 44, A. J. Menezes, P. C. van Oorschot, S. A. Vanstone, Handbook of Applied Cryptography, CRC Press, 1996. asymptotic approaches and directed attacks would, by definition, be unavailable under chance-based scenarios such as those relevant to Dembski's universal probability bound. As a result, Dembski's appeal to cryptography is best understood as referring to brute force attacks, rather than directed attacks.