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The mediocrity principle is the philosophical notion that "if an item is drawn at random from one of several sets or categories, it's likelier to come from the most numerous category than from any one of the less numerous categories" (Kukla 2009). The principle has been taken to suggest that there is nothing very unusual about the evolution of our Solar System, the Earth, humans, or any one nation. It is a heuristic in the vein of the Copernican principle, and is sometimes used as a philosophical statement about the place of humanity. The idea is to assume mediocrity, rather than starting with the assumption that a phenomenon is special, privileged or exceptional.
The mediocrity principle suggests, given the existence of life on Earth, that life typically exists on Earth-like planets throughout the universe. André Kukla criticizes the argument from mediocrity on two counts:
The first is that whatever prima facie
plausibility the principle of mediocrity may have is entirely dependent on the single case having been drawn at random. But the earth is not a randomly selected planet... The problem of randomness aside, the principle of mediocrity... is amenable to two drastically different readings, one of which is a probabilistic truism, the other a fallacy. The principle that's needed to underwrite [extraterrestrial intelligence] is the fallacious version. But the fallacy is obscured by virtue of its being confused with the truism. On one reading, the principle states that the single randomly drawn object is more likely to have come from the category that we know to be more numerous. This is the truism. If category A contains 3 elements and category B contains 1 element, then a random draw from the total population of 4 elements has a 3/4 probability of having come from A, and only a 1/4 probability of having come from B. This inference presupposes that we have antecedent knowledge of the relative numerosities of the classes A and B. In its [extraterrestrial intelligence] application, however, our antecedent knowledge and the inference we draw from it are reversed. We know that the random choice has come from A, and we infer from this that A is probably more numerous than B. For example, the classes A and B are "inhabitable planets that contain life" and "inhabitable planets that do not contain life," respectively, and the fact that our single examined case belongs to A is alleged to license the inference that A is probably more numerous than B (more vaguely, that the proportion of A's is not inconsiderable). This is an altogether more speculative inference than the first.
—André Kukla, Extraterrestrials: A Philosophical Perspective
Other uses of the heuristic 
David Deutsch argues that the mediocrity principle is not actually correct from a physical point of view, either in reference to our part of the universe or to our species. Deutsch refers to Stephen Hawking's quote that "The human race is just a chemical scum on a moderate-sized planet, orbiting around a very average star in the outer suburb of one among a hundred billion galaxies.", noting that our neighborhood in the universe is not typical (80% of the universe's mass is dark matter) and that a concentration of mass such as our solar system is an "isolated, uncommon phenomenon". He also argues with Richard Dawkins' opinion that humans, as result of natural evolution, are limited to the capabilities of our species — Deutsch responds that even though evolution did not give humans the ability to detect neutrinos, scientists can currently detect them, significantly expanding their capabilities beyond what is available as result of evolution.
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