The knowledge of error and vice, as Plato and St. Thomas have taught us, is itself good, itself an instrument of orderly education. Evil is not to be located in the intellect but in the will. Knowledge of error and vice is a necessary element in knowing the complete truth about anything. Aquinas wisely surrounded his famous proofs for the existence of God with the two most crucial arguments against such proofs, that from the existence of evil and that from the hypothesis that we can explain everything with our own intellects. And to make the point more forcefully, Aquinas proceeded to formulate these objections more concisely and accurately than anyone before or since has succeeded in doing. Indeed, no knowledge of truth is secure without an awareness of how multiple errors derive from and are related to truth.

- James Schall, On the Unseriousness of Human Affairs