Aristotle on Natural Law
Particular law is that which each community lays down and applies to its own members: this is partly written and partly unwritten. Universal law is the law of Nature. For there really is, as every one to some extent divines, a natural justice and injustice that is binding on all men, even on those who have no association or covenant with each other. It is this that Sophocles' Antigone clearly means when she says that the burial of Polyneices was a just act in spite of the prohibition: she means that it was just by nature.
Not of to-day or yesterday it is,
But lives eternal: none can date its birth.
And so Empedocles, when he bids us kill no living creature, says that doing this is not just for some people while unjust for others,
Nay, but, an all-embracing law, through the realms of the sky
Unbroken it stretcheth, and over the earth's immensity.
--Aristotle's Rhetoric 1.13
Here we should note: this is Aristotle's Rhetoric--it's designed for public speakers in general and for those giving speeches in courts in particular. It's about the kinds of arguments that can successfully be used in courts, and how to make them. For Aristotle, appealing to natural law to nullify a particular law is part of the whole purpose of his discussion of natural law. It allows the oppressed to appeal for justice--not only against unjust leaders, but against unjust laws and unjust constitutions.
Aristotle goes on to tell those who appear in court:
If the written law tells against our case, clearly we must appeal to the universal law, and insist on its greater equity and justice. We must argue that the juror's oath "I will give my verdict according to honest opinion" means that one will not simply follow the letter of the written law. We must urge that the principles of equity are permanent and changeless, and that the universal law does not change either, for it is the law of nature, whereas written laws often do change. This is the bearing of the lines in Sophocles' Antigone, where Antigone pleads that in burying her brother she had broken Creon's law, but not the unwritten law:
Not of to-day or yesterday they are,
But live eternal: none can date their birth.
Nor would I fear the wrath of any man
And brave God's vengeance for defying these.
We shall argue that justice indeed is true and profitable, but that sham justice is not, and that consequently the written law is not, because it does not fulfil the true purpose of law. Or that justice is like silver, and must be assayed by the judges, if the genuine is to be distinguished from the counterfeit. Or that the better a man is, the more he will follow and abide by the unwritten law in preference to the written.
--Aristotle's Rhetoric 1.15
It should not take a great deal of imagination to see why such a legal philosophy would appeal to Justice Thomas--a son of the segregated South.