This passage from Algernon Sidney (1622-1683) encapsulates an important part of the idea of the rule of law (or “written reason”), namely that it must be applied equally and impartially to all individuals and must not be subject to “mitigation or interpretation” by the ruler. It appeared in Sidney’s unpublished book Discourses Concerning Government which was used to charge, try, and execute him for treason in 1683:
’Tis not therefore upon the uncertain will or understanding of a prince, that the safety of a nation ought to depend. He is sometimes a child, and sometimes overburden’d with years. Some are weak, negligent, slothful, foolish or vicious: others, who may have something of rectitude in their intentions, and naturally are not incapable of doing well, are drawn out of the right way by the subtlety of ill men who gain credit with them. That rule must always be uncertain, and subject to be distorted, which depends upon the fancy of such a man. He always fluctuates, and every passion that arises in his mind, or is infused by others, disorders him. The good of a people ought to be established upon a more solid foundation. For this reason the law is established, which no passion can disturb. ’Tis void of desire and fear, lust and anger. ’Tis mens sine affectu [mind without passion], written reason, retaining some measure of the divine perfection. It does not enjoin that which pleases a weak, frail man, but without any regard to persons commands that which is good, and punishes evil in all, whether rich or poor, high or low. ’Tis deaf, inexorable, inflexible.
…Fortescue says plainly, the king cannot change any law: Magna Charta casts all upon the laws of the land and customs of England: but to say that the king can by his will make that to be a custom, or an ancient law, which is not, or that not to be so which is, is most absurd. He must therefore take the laws and customs as he finds them, and can neither detract from, nor add anything to them. The ways are prescribed as well as the end. Judgments are given by equals, per pares. The judges who may be assisting to those, are sworn to proceed according to law, and not to regard the king’s letters or commands. The doubtful cases are reserved, and to be referred to the parliament, as in the statute of 35 Edw. 3d concerning treasons, but never to the king. The law intending that these parliaments should be annual, and leaving to the king a power of calling them more often, if occasion require, takes away all pretence of a necessity that there should be any other power to interpret or mitigate laws. For ’tis not to be imagined that there should be such a pestilent evil in any ancient law, custom, or later act of parliament, which being on the sudden discover’d, may not without any great prejudice continue for forty days, till a parliament may be called; whereas the force and essence of all laws would be subverted, if under colour of mitigating and interpreting, the power of altering were allow’d to kings, who often want the inclination, and for the most part the capacity of doing it rightly. ’Tis not therefore upon the uncertain will or understanding of a prince, that the safety of a nation ought to depend. He is sometimes a child, and sometimes overburden’d with years. Some are weak, negligent, slothful, foolish or vicious: others, who may have something of rectitude in their intentions, and naturally are not incapable of doing well, are drawn out of the right way by the subtlety of ill men who gain credit with them. That rule must always be uncertain, and subject to be distorted, which depends upon the fancy of such a man. He always fluctuates, and every passion that arises in his mind, or is infused by others, disorders him. The good of a people ought to be established upon a more solid foundation. For this reason the law is established, which no passion can disturb. ’Tis void of desire and fear, lust and anger. ’Tis mens sine affectu [mind without passion], written reason, retaining some measure of the divine perfection. It does not enjoin that which pleases a weak, frail man, but without any regard to persons commands that which is good, and punishes evil in all, whether rich or poor, high or low. ’Tis deaf, inexorable, inflexible.
By this means every man knows when he is safe or in danger, because he knows whether he has done good or evil. But if all depended upon the will of a man, the worst would be often the most safe, and the best in the greatest hazard: Slaves would be often advanced, the good and the brave scorn’d and neglected. The most generous nations have above all things sought to avoid this evil: and the virtue, wisdom and generosity of each may be discern’d by the right fixing of the rule that must be the guide of every man’s life, and so constituting their magistracy that it may be duly observed. Such as have attained to this perfection, have always flourished in virtue and happiness: They are, as Aristotle says, governed by God, rather than by men, whilst those who subjected themselves to the will of a man were governed by a beast.
This being so, our author’s next clause, that “tho a king do frame all his actions to be according unto law, yet he is not bound thereunto, but as his good will, and for good example, or so far forth as the general law for the safety of the commonwealth doth naturally bind him,” is wholly impertinent. For if the king who governs not according to law, degenerates into a tyrant, he is obliged to frame his actions according to law, or not to be a king; for a tyrant is none, but as contrary to him, as the worst of men is to the best. But if these obligations were untied, we may easily guess what security our author’s word can be to us, that the king of his own good will, and for a good example, will frame his actions according to the laws; when experience instructs us, that notwithstanding the strictest laws, and most exquisite constitutions, that men of the best abilities in the world could ever invent to restrain the irregular appetites of those in power, with the dreadful examples of vengeance taken against such as would not be restrained, they have frequently broken out; and the most powerful have for the most part no otherwise distinguished themselves from the rest of men, than by the enormity of their vices, and being the most forward in leading others to all manner of crimes by their example.
Quotations 
