http://en.wikipedia.org/wiki/Goldilocks_Principle
Goldilocks Principle
The Goldilocks Principle states that something must fall within certain margins, as opposed to reaching extremes. It is used, for example, in the Rare Earth hypothesis to state that a planet must neither be too far away from, nor too close to, the sun to support life. Either extreme would result in a planet incapable of supporting life. Such a planet is colloquially called a "Goldilocks planet".
The Goldilocks Principle also applies in biomedicine; for instance, anti-thrombotic proteins and prothrombotic proteins exist in certain margins, and an extreme of either one would result in death.[citation needed]
The Goldilocks Principle comes from a children's story Goldilocks and the Three Bears in which a little girl found a house owned by three bears. Each bear owned a separate copy of many things, such as food, beds, etc. After testing each of the three, Goldilocks determined that one was always too much in one extreme (too hot, too large, etc.), one was too much in the opposite extreme (too cold, too small, etc.), and one was "just right."
