According to Occam's razor, all other things being equal, the simplest theory is the most likely to be true -- hence the importance of the concept of simplicity in epistemology. According to Thomas Aquinas, God is infinitely simple.

In MCS cognition theory, simplicity is the property of a domain which requires very little information to be exhaustively described. This is the contrary of complexity.

Simplicity is sometimes used as a synonym or euphemism for stupidity.