The 14th-century Franciscan friar and logician William of Ockham (or Occam, in the Latin spelling) once wrote "Pluralitas non est ponenda sine neccesitate." Known as Occam's Razor, this phrase translates to "entities should not be multiplied unnecessarily." In other words, the simplest explanation is best. Though this principle might seem obvious, it underpins virtually every scientific discovery ever made. For example, Einstein's theory of special relativity and the Lorentz ether theory both sought to explain phenomena related to the space-time continuum, such as how things tend to slow down the closer they get to moving at the speed of light. Lorentz's theory required a belief in "the ether," an invisible substance believed to connect everything in the universe, which had never been proven to exist. Such complications weren't necessary for Einstein's theory to work, so it's the one we use today. In everyday life, this is a good rule of thumb for anything that seems strange or inexplicable. Learn more about Occam's Razor and other philosophical princples, below.