Algernon Sidney argues that a law that is not just is not a law (1683)

SECTION 11: That which is not just, is not Law; and that which is not Law, ought not to be obeyed.

… the directive power of the law, which is certain, and grounded upon the inherent good and rectitude that is in it, is that alone which has a power over the conscience, whereas the coercive is merely contingent; and the most just powers commanding the most just things, have so often fallen under the violence of the most unjust men, commanding the most execrable villainies, that if they were therefore to be obeyed, the consciences of men must be regulated by the success of a battle or conspiracy, than which nothing can be affirmed more impious and absurd….

If this were so, the governments of the world might be justly called magna latrocinia; and men laying aside all considerations of reason or justice, ought only to follow those who can inflict the greatest punishments, or give the greatest rewards. But since the reception of such opinions would be the extirpation of all that can be called good, we must look for another rule of our obedience, and shall find that to be the law, which being, as I said before, sanctio recta, must be founded upon that eternal principle of reason and truth, from whence the rule of justice which is sacred and pure ought to be deduced, and not from the depraved will of man, which fluctuating according to the different interests, humors and passions that at several times reign in several nations, one day abrogates what had been enacted the other. The sanction therefore that deserves the name of a law, which derives not its excellency from antiquity, or from the dignity of the legislators, but from an intrinsick equity and justice, ought to be made in pursuance of that universal reason to which all nations at all times owe an equal veneration and obedience.