Section 3 of Ch. II of Bk. I of A Treatise of Eternal and Immutable Morality by Ralph Cudworth. This is page 249 of Bk II of L. A. Selby-Bigge's British Moralists.
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"Now the necessary Consequence of that which we have hitherto said is this, That it is so far from being true, that all Moral Good and Evil, Just and Unjust are meer Arbitrary and Factitious things, that are created wholly by Will; that (if we would speak properly) we must needs say that nothing is Morally Good or Evil, Just or Unjust by meer Will without Nature, because every thing is what it is by Nature, and not by Will. For though it will be objected here, that when God, or Civil Powers Command a Thing to be done, that was not before†1 obligatory or unlawful, the thing Willed or Commanded doth forthwith become†2 Obligatory, that which ought to be done by Creatures and Subjects respectively; in which the Nature of Moral Good or Evil is commonly Conceived to consist. And therefore if all Good and Evil, Just and Unjust be not the Creatures of meer Will (as many assert) yet at least Positive things must needs owe all their Morality, their Good and Evil to meer Will without Nature: Yet notwithstanding, if we well Consider it, we shall find that even in Positive Commands themselves, meer Will doth not make the thing commanded Just or Obligatory, or beget and create any Obligation to Obedience; but that it is Natural Justice or Equity, which gives to one the Right or Authority of Commanding, and begets in another Duty and Obligation to Obedience. Therefore it is observable, that Laws and Commands do not run thus, to Will that this or that thing shall become Just or Unjust, Obligatory or Unlawful; or that Men shall be obliged or bound to obey; but only to require that something be done or not done, or otherwise to menace Punishment to the Transgressors thereof. For it was never heard of, that any one founded all his Authority of Commanding others, and others Obligation or Duty to Obey his Commands, in a Law of his own making, that men should be Required, Obliged, or Bound to Obey him. Wherefore since the thing willed in all Laws is not that men should be Bound or Obliged to Obey; this thing cannot be the product of the meer Will of the Commander, but it must proceed from something else; namely, the Right or Authority of the Commander, which is founded in natural Justice and Equity, and an antecedent Obligation to Obedience in the Subjects; which things are not Made by Laws, but pre-supposed before all Laws to make them valid: And if it should be imagined, that any one should make a positive Law to require that others should be Obliged, or Bound to Obey him, every one would think such a Law ridiculous and absurd; for if they were Obliged before, then this Law would be in vain, and to no Purpose; and if they were not before Obliged, then they could not be Obliged by any Positive Law, because they were not previously Bound to Obey such a Person's Commands: So that Obligation to Obey all Positive Laws is Older than all Laws, and Previous or Antecedent to them. Neither is it a thing that is arbitrarily Made by Will, or can be the Object of Command, but that which either Is or Is not by Nature. And if this were not Morally Good and Just in its own Nature before any Positive Command of God, That God should be Obeyed by his Creatures, the bare Will of God himself could not beget an Obligation upon any to Do what he Willed and Commanded, because the Natures of things do not depend upon Will, being not things that are arbitrarily Made, but things that Are. To conclude therefore, even in Positive Laws and Commands it is not meer Will that Obligeth, but the Natures of Good and Evil, Just and Unjust, really existing in the World."