A rule is nonetheless capable of being a rule of law, though no reason can be given for it.
Aries Tanker Corporation v Total Transport Ltd (The Aries)  1 All ER 398, per Lord Wilberforce
… in the absence of an expressed constitutional or legal provision, it could not follow from the mere fact that a rule violated standards of morality that it was not a rule of law; and, conversely, it could not follow from the mere fact that a rule was morally desirable that it was a rule of law.
Positivism and the Separation of Law and Morals, H.L.A. Hart, 71 Harv. L. Rev. 593