Caroline Stephen, from Quaker Strongholds, the Fourth Edition, printed 1907.
"I believe that asceticism is in a very deep sense contrary to the real Quaker spirit, which desires in all things to abstain from any interference 'in the will of man' with Divine discipline and guidance, and which would, I believe, regard the idea of self-chosen exercises in mortification of the flesh with the same aversion as it entertains for pre-arranged forms of worship. Friends, no doubt, have often believed themselves required to submit to the adoption of plain dress 'in the cross' to natural inclination, and have felt it a valuable exercise to do so; but the plainness was not devised for that purpose, but chosen (or rather, as Friends would say, they were led into it by the Truth) because of its inherent suitableness and rightness. It is an outcome of the instinctively felt necessity of subordinating everything to principle. Its chief significance is that of a protest against bondage to passing fashions, and for this reason it is a settled costume. It is also felt that our very dress should show forth that inward quietness of spirit which does not naturally tend towards outward adornment, and the Friends' recognized dress is therefore one of extreme sobriety in colour and simplicity in form.
"It is a significant fact that there is really no such thing as a precisely defined Quaker costume. The dress certainly looks precise enough in itself, and to the naked eye of the outside observer it may appear to present an undeviating unformity; but it is really not a uniform in the sense in which a nun's or a soldier's dress is a uniform. It is in all respects a growth, a tradition, a language; and it is subject to constant though slow modification. Any perfectly unadorned dress of quiet colour, without ornament or trimming, if habitually worn, is in fact to all intents and purposes the Quaker costume, though one or two details have by a sort of accident acquired a traditional meaning as a badge, which one may adopt or not according to one's feelings about badges. Some Friends now-a-days object on principle to anything of the kind. Others still see a 'hedge' or shelter in them. Others, again, feel that they serve a useful and innocent purposein enabling Friends readily to recognize one another, and that it is not amiss for them to be easily recognized even by outsiders. But ithe one important matter of principle which the Society as a body have recognized, is that it is a waste of time and money for which Christian women can hardly fail to find better employment, to condescend to be perpetually changing the fashion of one's garments in obedience to the caprice or the restlessness of the multitude. 'Plain Friends' are those who are resolved to dress according to the settled principles which commend themselves to their own minds, not enslaving them to passing fashion.
"It is easy to say that they do but exchange one bondage for another. That may, indeed, have been the case at times, and may even still be so in some families or meetings. But the crystallizing into rigid formality, though a possible tendency, is no real part of the true Quaker ideal. My own strong feeling is that the adoption of a settled costume, at any rate in mature life and from conviction, is not only the right and most dignified course on moral grounds, but also that it has in actual experience afforded one more proof of the truth that the lower aims of life can thrive only in proportion as they are kept in subordination to the higher. The freedom from the necessity of perpetual changes, which commends itself to Friends as suitable to the dignity of 'women professing godliness' has also the lower advantage of admitting a gradual bringing to perfection of the settled costume itself. We all know how exquisite, within its severely limited range, can be the result. The spotless delicacy, the precision and perfection of plain fine needlework, the repose of the soft tints, combine, in the dress of some still lingering representatives of the old school of Quakerism, to produce a result whose quiet beauty appeals to both the mind and theye with a peculiar charm. I cannot think that such mute eloquence is to be despised; or that their very dress shall speak a language of quietness, gentleness and purity--that it shall be impressed even with a touch of eternity."
(An Exhortation to set up Women's Meetings) AND so, that none may stand idle out of the Vineyard, and out of the Service, and out of their Duty; for such will Talk and Tattle, and Judge with Evil Thoughts, of what they in the Vineyard say and do: And therefore the Power of the Lord God calls in all, into their Duty, into their service, in their Place, in Vertue and Righteousness, and into the Wisdom and Power of God. . . . Therefore train up your young Women to know their Duty in ...
I am not Amish or Mennonite, but some people who come to my website are interested in knowing more about these groups. I can recommend these books as authoritative and relatively inexpensive sources of further information.