“The validity of demonstrably wrong law cannot conceivably be justified. However, any answer to the question of the purpose of law other than by enumerating the manifold partisan views about it has proved impossible— and it is precisely on that impossibility of any natural law, and on that alone, that the validity of positive law may be founded. At this point relativism, so far only the method of our approach, enters our system as a structural element.
Ordering their living together cannot be left to the legal notions of the individuals who live together, since these different human beings will possibly issue contradictory directions. Rather, it must be uniformly governed by a transindividual authority. Since, however, in the relativistic view of reason and science are unable to fulfill that task, will and power must undertake it. If no one is able to determine what is just, somebody must lay down what is to be legal.”

General Theory of Law and State (1949), I. The Concept of Law, A. Law and Justice, a. Human Behavior as the Objects of Rules

1860s, Letter to James C. Conkling (1863)
Context: But the proclamation, as law, either is valid, or is not valid. If it is not valid, it needs no retraction. If it is valid, it can not be retracted, any more than the dead can be brought to life. Some of you profess to think its retraction would operate favorably for the Union. Why better after the retraction, than before the issue? There was more than a year and a half of trial to suppress the rebellion before the proclamation issued, the last one hundred days of which passed under an explicit notice that it was coming, unless averted by those in revolt, returning to their allegiance. The war has certainly progressed as favorably for us, since the issue of the proclamation as before. I know as fully as one can know the opinions of others, that some of the commanders of our armies in the field who have given us our most important successes, believe the emancipation policy, and the use of colored troops, constitute the heaviest blow yet dealt to the rebellion; and that, at least one of those important successes, could not have been achieved when it was, but for the aid of black soldiers. Among the commanders holding these views are some who have never had any affinity with what is called abolitionism, or with republican party politics; but who hold them purely as military opinions. I submit these opinions as being entitled to some weight against the objections, often urged, that emancipation, and arming the blacks, are unwise as military measures, and were not adopted, as such, in good faith.

"Five Minutes of Legal Philosophy" (1945)

Der Relativismus in der Rechtsphilosophie (1934).

De Re Publica [Of The Republic], Book III Section 22; as translated by Francis Barham
Variant translations:
True law is right reason in agreement with nature; it is of universal application, unchanging and everlasting; it summons to duty by its commands, and averts from wrongdoing by its prohibitions. And it does not lay its commands or prohibitions upon good men in vain, though neither have any effect on the wicked. It is a sin to try to alter this law, nor is it allowable to attempt to repeal any part of it, and it is impossible to abolish it entirely. We cannot be freed from its obligations by senate or people, and we need not look outside ourselves for an expounder or interpreter of it. And there will not be different laws at Rome and at Athens, or different laws now and in the future, but one eternal and unchangeable law will be valid for all nations and all times, and there will be one master and ruler, that is, God, over us all, for he is the author of this law, its promulgator, and its enforcing judge. Whoever is disobedient is fleeing from himself and denying his human nature, and by reason of this very fact he will suffer the worst penalties, even if he escapes what is commonly considered punishment.
As translated by Clinton W. Keyes (1928)<!-- ; in De Re Publica, De Legibus (1943), p. 211 -->
Context: There is a true law, a right reason, conformable to nature, universal, unchangeable, eternal, whose commands urge us to duty, and whose prohibitions restrain us from evil. Whether it enjoins or forbids, the good respect its injunctions, and the wicked treat them with indifference. This law cannot be contradicted by any other law, and is not liable either to derogation or abrogation. Neither the senate nor the people can give us any dispensation for not obeying this universal law of justice. It needs no other expositor and interpreter than our own conscience. It is not one thing at Rome and another at Athens; one thing to–day and another to–morrow; but in all times and nations this universal law must for ever reign, eternal and imperishable. It is the sovereign master and emperor of all beings. God himself is its author,—its promulgator,—its enforcer. He who obeys it not, flies from himself, and does violence to the very nature of man. For his crime he must endure the severest penalties hereafter, even if he avoid the usual misfortunes of the present life.

Individualism and Socialism (1933)

Source: The Logic of Scientific Discovery (1934), Ch. 1 "A Survey of Some Fundamental Problems", Section I: The Problem of Induction http://dieoff.org/page126.htm p. 27

"The Prince's Dog", p. 201
The Dyer's Hand, and Other Essays (1962)

Source: Letters from the Earth: Uncensored Writings
Source: Mark Twain's Notebook (1935), p. 381