A theory posits causes for observed effects, though the true causes be unknown.

A valid theory is one that provides a close match with the observed effects or facts, and explains how it can be conclusively proved true or false in all cases everywhere. A valid theory must be capable of being tested and the tests must be capable of independent verification. A strong theory is one resistant to attack from any direction.

No theory is absolutely certain, but a theory that has withstood the tests of time acquires higher degree of certainty the longer it survives.