The theory of evolution by natural selection is a scientific theory that is substantiated by empirical evidence. In science, a theory is defined as a plausible or scientifically acceptable principle used to explain phenomena. In order to become widely accepted, theories must be substantiated with evidence that is obtained in a rational and methodical manner. Scientific theories are reliable and rigorous and are not to be confused with the colloquial usage of the word theory to refer to speculation and unproven ideas.