Mediocrity principle

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

  • Fossil evidence supported by genetics concluding that all humans have a common ancestor about 100,000 years ago and that they share a common ancestor with chimpanzees about six million years ago. Therefore humans are part of the biosphere, not above it or unique to it.
  • Humans share about 98% of their DNA with chimpanzees.
  • The answering of Schrödinger's question What is Life? through the discovery of the double-helix structure of DNA and the reduction of life to organic chemisty, negating the vitalism of previous centuries.
  • Francis Crick's "Astonishing Hypothesis" suggests that consciousness is simply the function of the brain.
  • When the Human Genome Project released its findings in 2003, it was discovered that the human genome only has 24,000 genes. As recently as the 1990s, humans were considered so complex as to have about 300,000 genes.
  • Evolutionary psychology is discovering the limits to human rationality, cognition and moral sense with fMRI scans, economic and political studies.
  • Edwin Hubble discovered the universe is a lot larger than humans first thought and James Hutton discovered the Earth is a lot older. The Hubble Deep Field is a long exposure of thousands of galaxies, making it one of the best pictorial representations of the principle of mediocrity.


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[edit] The Earth is an unexceptional planet

The traditional formulation of the Copernican mediocrity principle is usually played out in the following way: Ancients once thought that the 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 the solar system was not at the center of the 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 the Sun is unusual in having planets. In short, Copernican mediocrity is a series of astronomical findings that the 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.

[edit] Critics of the 'ordinary earth' mediocrity principle

Main article: Rare Earth hypothesis

In arguing that human's 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. Supporters of the Rare Earth hypothesis 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]

Supporters of the Rare Earth hypothesis argue that not only is the cosmos as a whole finely tuned for life, but within it, the Earth's peculiarities make it an extremely special 'Pale Blue Dot'. Rare Earth: Why Complex Life is Uncommon in the Universe made similar claims in 2000. These objections to the mediocrity principle are based on the hypothesis that the following planetary advantages are both extremely uncommon and absolutely essential for life:

  • Earth occupies the perfect orbit around a non-binary, metal-rich star (the Sun) with stable radiation over an ideal frequency spectrum. If the Sun was larger, it would burn too quickly for life to evolve and if it was smaller, the Earth would need to be closer, making it tidally locked.
  • Earth is a silicate rock with the prerequisite mass, plate tectonics, and iron core to protect developing life from radiation.
  • Jupiter and the other large outer planets shield the Earth from asteroids without destabilizing its orbit as well as shuttling water-rich comets from the outer solar system to the inner.
  • Earth has the perfect amount of water for a long-term active hydrosphere.
  • The Moon 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.
  • The 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).
  • Earth's orbital and temperature stability over billions of years is exceedingly rare, as is its insulation from cataclysmic events.

Advances in detection of extrasolar planets may validate or disprove many of these hypotheses in the near future.

[edit] 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 belief is that 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. Although this is a popular interpretation of history and of man's position in the cosmos, it is not historically accurate. Medieval theologians, most vividly illustrated in Dante's Divine Comedy, viewed the heavens as perfect, and Earth (and humans) as the dregs (rather than the pinnacle) of creation. Thus it seemed that Copernicus was actually promoting rather than demoting the Earth by removing it from the "basement", and the paradigm shift was of a very different character.

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