The Fourth Way of Thomas Aquinas, known colloquially as the argument from perfection and the argument from gradation has numerous formulations. One easy way to state the argument is like this:
1. A flaw or degree in something cannot be known unless there is a standard of perfection for it. (Premise)
2. There are flaws in truth-claims and degrees of goodness. (Premise)
3. Therefore, there is some Supreme Truth and Supreme Goodness that is the standard of perfection by which the imperfection of other things can be intelligible. (From 1 and 2)
In support of (1), C.S. Lewis is famous for stating that a man would have no idea what a crooked line looks like unless he already knew what a straight line looks like. The fact that there is deviation entails that a thing must deviate from some standard of perfection. Given premise (2), which appears obviously true, it follows that there is some standard of perfection for truth and goodness, which we call God.