Aquinas famously said that the first principle of practical reason is that “good is to be done and evil avoided.” Modern Christians are liable to retort, “Well that’s not very useful.” For many today, one of the greatest objections to natural revelation is its lack of precision and certainty. In troubled times, seeking a clear moral compass, Christians cannot understand why we would want to rely on the vague and uncertain guidance of natural law when we have the clear and certain moral teaching of Scripture. In this lecture, I will seek to turn this objection on its head, arguing that, on the contrary, the uncertainty of natural law is its greatest asset. The moral life is always and unavoidably one of uncertainty, requiring the cultivation of profound discernment and wisdom in how to do good and avoid evil in ever-shifting and often murky circumstances. I will argue that the intellectual and spiritual virtues required in discerning and applying what nature teaches—preeminently the neglected virtue of prudence—are precisely those virtues that the Apostle Paul calls disciples of Christ to cultivate as they live out obedience to God’s Word.