Mediocrity Principle - Extraterrestrial Life

Extraterrestrial Life

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 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 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