QuestionsWhat makes something right?The natural law
What makes something right?Nature or conscience

The natural law

Thomas Aquinas 1225 · 1274Faith and reason cannot contradict each other.
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What it claims

Preserving life, caring for one's children, seeking the truth and living at peace with others are precepts of the natural law, valid for all.

Why

What our nature inclines us toward, reason grasps as good: staying alive, raising children, knowing the truth.

In short

"The natural law" is an argument by Thomas Aquinas (Summa theologiae, I-II, q.94, a.2) on the question "What makes something right?" (answer: nature or conscience). Its claim: Preserving life, caring for one's children, seeking the truth and living at peace with others are precepts of the natural law, valid for all. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P4). AskPhilosophy presents 4 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

Summa theologiae, I-II, q.94, a.2
Valid with an implicit premise

Are there moral rules valid for all human beings, believers or not? Aquinas says yes: reason reads them in our natural inclinations, starting from a principle everyone accepts.

P1The good is what all things seek. Hence the first precept: good is to be done and pursued, and evil avoided.
P2Whatever humans are naturally inclined to, reason grasps as good, and so as something to pursue.
P3Humans are naturally inclined to preserve their life (like every being), to mate and raise their children (like every animal), to know the truth and to live in society (as rational beings).
P4 · implicit premiseWhat reason naturally grasps as good really is good: our natural inclinations, ordered by reason, show what is good for humans, not merely what they happen to want. For Aquinas, this is because they are the imprint in us of God's eternal law.
Therefore preserving life, caring for one's children, seeking the truth and living at peace with others are precepts of the natural law, valid for all.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P4, the computer finds a case where we are drawn toward life or social life, where reason sees them as goods, but where that perception is mistaken: nothing then obliges us to pursue them.
Valid with P4. Without it, we move from "what we are inclined to" to "what we ought to do" without justification: the jump Hume would criticise in all ethics of this kind. In Aquinas's system P4 is not hidden: it follows from the idea that our inclinations share in the eternal law (I-II, q.91, a.2). But it does not appear in the argument itself, and it presupposes a Creator or, at least, a human nature directed toward ends. Twentieth-century Thomists (Germain Grisez, John Finnis) answer Hume that Aquinas does not deduce a duty from a fact: the basic goods (life, knowledge, friendship) are self-evident to practical reason, like first principles.
Analogy. A user guide for a seed: its nature already shows what is good for it (water, light, soil). Aquinas thinks human nature likewise shows its goods, and that reason can read that guide. The question remains: who wrote the guide, and is there one at all?

Objections

4

Each objection is a conversation: click to see who replies and where the debate stands.

Objection anticipated by Aquinas (q.94, a.4, objection 3) attacks premise 3

Humans do not all have the same inclinations: some are drawn to pleasures, others to honours, others to other things. So there cannot be one natural law for all.

Thomas d'Aquin replies
Thomas d'Aquin
What is common to all is that reason must direct all these inclinations. The variety of desires does not rule out a single rule: order them according to reason.
Objection anticipated by Aquinas (q.94, a.4: the diversity of peoples) attacks premise 2

Peoples do not judge alike. Aquinas gives the example himself: according to Julius Caesar, the ancient Germans did not consider plunder unjust. If reason naturally saw the good, everyone would see the same thing.

Thomas d'Aquin replies
Thomas d'Aquin
The first principles are the same for all and known to all. It is the detailed applications that can vary or be obscured, by passion, bad custom or a bad disposition.
Hume (no "ought" from an "is") 1740 · attacks premise 4

From what is (we are drawn to something) we cannot deduce what ought to be (we must pursue it). A natural inclination remains a fact; an extra reason is needed to make it a duty. Hume's text (Treatise of Human Nature, Book III) is not yet in our corpus.

no reply
summary: their text is not yet in our corpus
No reply possible: objection raised in 1740, after the author's death (1274).
Darwin and evolutionary biology 1871 · attacks premise 4

Darwin explains the moral sense by evolution (The Descent of Man, 1871). Our inclinations come from natural selection: they helped our ancestors survive and reproduce. Nothing guarantees they point to what is good; some (aggression, distrust of strangers) are natural and yet bad.

no reply
summary: their text is not yet in our corpus
No reply possible: objection raised in 1871, after the author's death (1274).
What today's voices say
Defend it
Bishop Robert BarronHe holds that natural law and virtue ethics fit Catholic morality best.