If, for these and other such rules, you insist upon having positive Scripture injunction, you will find none. Tradition will be held forth to you as the originator of them, custom as their strengthener, and faith as their observer. That reason will support tradition, and custom, and faith, you will either yourself perceive, or learn from some one who has. Meanwhile you will believe that there is some reason to which submission is due. I add still one case more, as it will be proper to show you how it was among the ancients also.
Among the Jews, so usual is it for their women to have the head veiled, that they may thereby be recognised. I ask in this instance for the law. I put the apostle aside. If Rebecca at once drew down her veil, when in the distance she saw her betrothed, this modesty of a mere private individual could not have made a law, or it will have made it only for those who have the reason which she had. Let virgins alone be veiled, and this when they are coming to be married, and not till they have recognised their destined husband.
If Susanna also, who was subjected to unveiling on her trial, furnishes an argument for the veiling of women, I can say here also, the veil was a voluntary thing. She had come accused, ashamed of the disgrace she had brought on herself, properly concealing her beauty, even because now she feared to please. But I should not suppose that, when it was her aim to please, she took walks with a veil on in her husband's avenue. Grant, now, that she was always veiled. In this particular case, too, or, in fact, in that of any other, I demand the dress-law.
If I nowhere find a law, it follows that tradition has given the fashion in question to custom, to find subsequently (its authorization in) the apostle's sanction, from the true interpretation of reason. This instances, therefore, will make it sufficiently plain that you can vindicate the keeping of even unwritten tradition established by custom; the proper witness for tradition when demonstrated by long-continued observance. But even in civil matters custom is accepted as law, when positive legal enactment is wanting; and it is the same thing whether it depends on writing or on reason, since reason is, in fact, the basis of law.
But, (you say), if reason is the ground of law, all will now henceforth have to be counted law, whoever brings it forward, which shall have reason as its ground. Or do you think that every believer is entitled to originate and establish a law, if only it be such as is agreeable to God, as is helpful to discipline, as promotes salvation, when the Lord says, “But why do you not even of your own selves judge what is right?” And not merely in regard to a judicial sentence, but in regard to every decision in matters we are called on to consider, the apostle also says, “If of anything you are ignorant, God shall reveal it unto you;” he himself, too, being accustomed to afford counsel though he had not the command of the Lord, and to dictate of himself as possessing the Spirit of God who guides into all truth.
Therefore his advice has, by the warrant of divine reason, become equivalent to nothing less than a divine command. Earnestly now inquire of this teacher, keeping intact your regard for tradition, from whomsoever it originally sprang; nor have regard to the author, but to the authority, and especially that of custom itself, which on this very account we should revere, that we may not want an interpreter; so that if reason too is God's gift, you may then learn, not whether custom has to be followed by you, but why.
Source: De Corona (The Chaplet) (New Advent)