Occam's razor

"Pluralitas non est ponenda sine necessitate" which is translated: "plurality should not be posited without necessity."

The principle
This principle was named after William of Ockham (or Occam), a fourteenth century Franciscan and philosopher, who frequently used it, though he was not its originator. In modern English, it can be restated as: a simpler explanation is to be preferred to a more complicated one.

Proper Use
An example of the proper application of this principle is the choice of Kepler's laws of planetary motion, in preference to the Ptolemaic cycle and epicycles, as an explanation of the movement of the heavenly bodies. The Ptolemaic hypothesis required constant adjustment with further epicycles in order to explain new observations while preserving the philosophical assumption that all movements in the heavens were perfect circles. Kepler's laws provided a far simpler theory, which was both supported by observation and could be described by a single mathematical formula. Kepler's was therefore to be preferred.

Atheistic Use
The principle is frequently misused by atheists, who claim that the entire spiritual realm is an unnecessary hypothesis. In doing so, they ignore all human spiritual experience, as well as having to violate basic scientific laws, such as thermodynamics. Their mistake is so to simplify their hypotheses that they no longer explain anything!

Related References

 * The Skeptics' Dictionary
 * Wikipedia