From the reality and binding force of moral obligation to a ground beyond nature. It comes in two distinct forms: Newman's argument from conscience, and Kant's practical postulate — which is expressly not a theoretical proof. Its classic objections are Plato's Euthyphro dilemma and Hume's is/ought gap.
The numbered form is ours, after Newman's Grammar of Assent and Kant's practical postulate; cf. the SEP, "Moral Arguments for the Existence of God" — the words themselves are the philosophers', below.
Now it was seen to be a duty for us to promote the summum bonum; consequently it is not merely allowable, but it is a necessity connected with duty as a requisite, that we should presuppose the possibility of this summum bonum; and as this is possible only on condition of the existence of God, it inseparably connects the supposition of this with duty; that is, it is morally necessary to assume the existence of God.
If, as is the case, we feel responsibility, are ashamed, are frightened, at transgressing the voice of conscience, this implies that there is One to whom we are responsible, before whom we are ashamed, whose claims upon us we fear … These feelings in us are such as require for their exciting cause an intelligent being … If the cause of these emotions does not belong to this visible world, the Object to which his perception is directed must be Supernatural and Divine.
Source — John Henry Newman, An Essay in Aid of a Grammar of Assent, ch. 5
The point which I should first wish to understand is whether the pious or holy is beloved by the gods because it is holy, or holy because it is beloved of the gods.
God alone is good essentially. For everything is called good according to its perfection … This triple perfection belongs to no creature by its own essence; it belongs to God only, in Whom alone essence is existence … Hence it is manifest that God alone has every kind of perfection by His own essence; therefore He Himself alone is good essentially.
In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surprized to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence.
Source — David Hume, A Treatise of Human Nature, Book III, Part I
Start with a plain example. Torturing a child for fun is wrong — and not just wrong-for-me, or wrong-in-our-culture. If an entire society approved of it, we would say the society was mistaken, not that the wrongness had gone away. That stubborn sense — that some things are really binding on everyone, whether they like it or not — is what the argument builds on.
The hinge is where that bindingness comes from. Newman's route is conscience: notice that when you do wrong you feel not just regret but shame and dread, the way you feel before a person you have let down — and you feel it even when no other human being knows. A mere rule, or a passing feeling, cannot look back at you; a person can. So conscience, Newman argues, carries the trace of Someone we answer to.
Two very different challenges meet this. Plato's Euthyphro asks a sharp question that cuts both ways: is a thing good because God commands it — in which case goodness looks arbitrary, since God could as well have commanded cruelty — or does God command it because it is already good, in which case goodness stands on its own and does not need God? And Hume pointed out a logical gap: from any set of facts about how things are, you cannot, by pure logic, derive a conclusion about how things ought to be — so no description of the world, God included, by itself yields an obligation.
Defenders answer the Euthyphro dilemma with a third option it leaves out. On classical theism God is not one more being who happens to issue commands — which would make them arbitrary — nor does he look up to a standard of goodness above himself. Rather, God simply is goodness: "God alone is good essentially," in Aquinas's words. So the good is neither arbitrary nor independent of God, because the standard of goodness is God's own unchanging nature, and his commands flow from what he is. (Robert Adams's modern divine-command theory develops the same move: goodness is likeness to God, and obligation is the command of a God who is himself loving.) Whether that dissolves the dilemma or merely relocates it is, as ever, left to the reader.
Kant grants much of this. He rejects the theoretical arguments — including this one, taken as a demonstration. But he holds that acting morally commits us to hope that goodness and happiness finally meet, and that this hope only makes sense if we postulate a God to secure it: not a proof of God's existence, but a God we are rationally bound to assume in order to live morally. Whether obligation is real, and whether it needs a ground beyond us, is where the argument lives.
Written for this library — not a quoted source. It reports how the premise is argued and answered; the philosophers' own words are above.
Accept that binding moral obligation needs a ground beyond nature, and Plato's Euthyphro presses the follow-up: if right and wrong depend on God's commands they look arbitrary; if they do not, morality seems to stand without God after all.
Plato, Euthyphro (hosted below); the modern divine-command reply: Robert Adams ↗
The modern divine-command answer locates the good in God's nature rather than in his commands, so that what is commanded is neither arbitrary nor answerable to a standard above him.
Robert Adams, Finite and Infinite Goods (1999) ↗
Deny that morality is objectively binding — treat it as human sentiment or evolved convention, as Hume and later Mackie do — and the argument has nothing left to stand on; but many find they cannot really believe that torturing the innocent is merely disliked, rather than wrong.
David Hume (hosted below); J. L. Mackie, Ethics: Inventing Right and Wrong (1977) ↗
Accept that moral obligation is real and objective, and this argument leads to a moral lawgiver — a source of obligation beyond nature, not yet the full God of classical theism.
The modern revivals and their critics are under copyright, so we don't host them — the reading doesn't stop at the public-domain record. Follow it at the sources:
You have read the premise, the objection, and the cost of each move. The library draws no conclusion.