Mediocrity principle

From Academic Kids

The mediocrity principle is the notion in the philosophy of science that there is nothing special about Earth, and by implication the human race. It is a Copernican principle, used either as a heuristic about Earth's position or a philosophical statement about the place of humanity.


The Earth is a not-very-exceptional planet

The traditional formulation of the Copernican mediocrity principle is usually played out in the following way: Ancients once thought that Earth was at the center of the solar system, but Copernicus proposed that the Sun was at the center. This heliocentric view was confirmed a hundred years later by Galileo, who demonstrated with a telescope that Jupiter's Moons orbited Jupiter and that Venus must orbit the Sun. In the 1930s, RJ Trumpler found that our solar system was not at the center of our Milky Way Galaxy (as Jacobus Kapteyn claimed), but 56% of the way out to the rim of the galaxy's core. In the mid Twentieth Century, George Gamow (et al.) showed that although it appears that our Galaxy is at the center of an expanding universe (in accordance with Hubble's law), every point in space experiences the same phenomenon. And, at the end of the Twentieth Century, Geoff Marcy and colleagues discovered that extrasolar planets are quite common, putting to rest the idea that our Sun is unusual in having planets. In short, the Copernican Mediocrity is the series of astronomical findings that Earth is a relatively ordinary planet orbiting a relatively ordinary star in a relatively ordinary galaxy which is one of countless others in a giant universe, possibly within an infinite multiverse.

Critics of the 'ordinary earth' mediocrity principle

In arguing that our planet, evolution, civilization, and technology are unexceptional SETI advocates invoke the mediocrity principle as a strong reason (via prior probability) to expect abundant extraterrestrial signals. For instance, Carl Sagan used the principle to argue that "there might be one million civilizations in the Milky Way. The failure to find such signals or evidence is taken by some as a disconfirmation of the mediocrity principle. (The lack of contact is interpreted more often as a scarcity of human-like intelligence than a scarcity of Earth-like planets, but a scarcity of either could be considered refutation of the mediocrity principle, depending on whether the principle is applied strictly to the planet or, more loosely, to its inhabitants.)

Denying the mediocrity principle is very similar to affirming the Rare Earth hypothesis; for example, Gonzalez and Richards (2004) present the case for Earth's uniqueness, in their book The Privileged Planet. They claim:

"Not that we are at the 'center' of the universe, but rather at the best location for complex life to flourish and to observe what is beyond us." [1] (

They argue that not only is the cosmos as a whole finely turned for life, but within it Terran peculiarities make it an extremely special 'Pale Blue Dot'. Rare Earth made similar claims in 2000. These attacks on the principle are based on the following planetary advantages, all alleged to be uncommon & essential for life:

  • Earth occupies the perfect orbit around a non-binary, metal-rich star (the Sun) with stable radiation over an ideal spectrum.
  • Earth is a silicate rock with the prerequisite mass, Plate tectonics, and iron core to protect developing life.
  • Earth's Jovian neighbors shield it from asteroids but don't destabilize its orbit.
  • Earth has the perfect amount of water for a long-term active hydrosphere.
  • Earth's moon (Luna) is anomalously massive, creating large oceanic tides, and stabilizing the Earth's axial tilt. According to Jacques Laskar's calculations this critical feature is otherwise impossible to achieve.
  • Earth's location within the galaxy is rare and important: "Not in the center of the galaxy, not in a globular cluster, not near an active gamma ray source, not in a multiple-star system, or near a pulsar, or near stars too small, too large, or soon to go supernova." (Rare Earth page 282).
For more details see: Rare Earth hypothesis

As a philosophical statement

There is a stronger, philosophical version of the mediocrity principle. This associates the Renaissance with greater openness to radical ideas. The Roman Catholic dogma of the day, with regards to the place of Earth in the cosmos, was that if God made man in God's image and that this was God's most perfect creation, then there was only one logical place to put this most perfect creation—at the center of the Universe.

Therefore, Copernicus's suggestion that Earth was not the center of the entire Universe, implied the theological conclusion that man was not God's most perfect creation.

This philosophical interpretation has been criticized by some historians who claim that theologies of the day viewed the heavens as perfect, and Earth (and humans) as the dregs (rather than the pinnacle) of creation. In this view, Copernicus was actually promoting rather than demoting the Earth by removing it from the "basement", and the paradigm shift didn't really occur.

See also


  • Gonzalez, Richards, The Privileged Planet: How Our Place in the Cosmos is Designed for Discovery 2004, Regnery Publishing, ISBN: 0895260654
  • Peter Ward and Donald Brownlee. Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus Books. January 2000. ISBN 0387987010 (See discussion in: Rare Earth hypothesis.)

External links


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