The most famous version of Natural Law Theory in Ethics is represented by St. Thomas of Aquinas’ adaptation of Aristotle’s teleological account of nature wherein every natural kind has an established telos (end, goal, or purpose) determined by the kind of thing it is. For Aristotle, everything, particularly in the terrestrial world, has a purpose or function which is the reason for its existence. Thus, the “good” for that thing is to fulfill its function.

St. Thomas adapts this teleology with the additional premise that the natural order is established by the will of God (e.g., nature is the way it is because God wills it to be that way). Applying this argument to humans, we can say that the good for humans is to act in accordance with the purpose God has set out for us (understood universally, not individually).

If this argument holds, morality will be identifiable through reason (which is peculiar to human nature) and will not require a special divine revelation (see Divine Command Theory).

One might also classify Stoic Ethics as a form of Natural Law Theory since the Stoics held that the universe’s order (kosmos) is a biproduct of the Logos (sometimes thought of in personified terms, though not necessarily). However, the goal of Stoic ethics is to achieve a state of apatheia (being untroubled by passion), as opposed to just performing one’s function as a human.