If the assistance of Mothers be, as I have already affirm'd it is, necessary to the right forming of the Minds, and regulating of the Manners of their Children; I am not in the wrong in reckoning (as I do) that this care is indispensibly a Mothers Duty. Now it cannot, I think, be doubted, but that a Mothers Concurrence and Care is thus necessary, if we consider that this is a work which can never be too soon begun, it being rarely at all well performed, if not betimes undertaken; nothing being so effectual to the making Men vertuous, as to have good Habits and Principles of Vertue establish'd in them before the Mind is tainted with any thing opposite or prejudicial hereunto. Those therefore must needs much over-look the chief Business of Education, or have little consider'd the Constitution of Humane Nature, that reckon for nothing the first eight or ten Years of a Boys Life; an Age wherein Fathers, who seldom are able to do it at any time, can neither charge themselves with the care of their Children, nor be the watchful inspectors of those that they must be trusted to; who usually and unavoidably by most Parents, are a sort of People far fitter to be Learners than Teachers of the Principles of Vertue and Wisdom; the great Foundation of both which consists in being able to govern our Passions, and subject our Appetites to the direction of our Reason: A Lesson hardly ever well learnt, if it be not taught us from our very Cradles. To do which requires no less than a Parents Care and Watchfulness; and therefore ought undoubtedly to be the Mothers business to look after, under whose Eye they are. An exemption from which, Quality (even of the highest degree) cannot give; since the Relation between the Mother and Child is equal amongst all Ranks of People. And it is a very preposterous Abuse of Quality to make it a pretence for being unnatural. This is a Truth which perhaps would displease many Ladies were it told them, and therefore, probably, it is that they so seldom hear it: But none of them could be so much offended with any one for desiring hereby to restrain them from some of their expensive and ridiculous Diversions, by an employment so worthy of Rational Creatures, and so becoming of maternal tenderness, as it is just to be with them for neglecting their Children: A Fault that women of Quality are every way too often guilty of, and are perhaps more without excuse for, than for any other that they are ordinarily taxable with. For tho' it is to be fear'd that few Ladies (from the disadvantage of their own Education) are so well fitted as they ought to be, to take the care of their Children, yet not to be willing to do what they can herein, either as thinking this a matter of too much pains for them, or below their Condition, expresses so senseless a Pride, and so much want of the affectionate and compassionate Tenderness natural to that Sex and Relation, that one would almost be tempted to question whether such Women were any more capable of, than worthy to be the Mothers of Rational Creatures.(pp. 76-77)
Masham's primary argument in Occasional Thoughts is that, because mothers are, in their own way, the first and most formative educational influences, it is essential for women to be brought up well-educated and treated as rational creatures. We can call this the Lockean argument for the better education of women. This argument for the better education of women is an adaptation of Lockean principles of education:
In Mr. L---- s excellent _Treatise of Education_, he shews how early and how great a Watchfulness and Prudence are requisite to the forming the Mind of a Child to Vertue; and whoso shall read what he has writ on that Subject, will, it is very likely, think that few Mothers are qualify'd for such an undertaking as this: But that they are not so is the Fault which should be amended: In the mean time nevertheless, their presum'd willingness to be in the right, where the Happiness of their Children is concerned in it, must certainly inable them, if they were but once convinc'd that this was their Duty, to perform it much better than such People will do, who have as little Skill and Ability for it as themselves; and who besides, that they rarely desire to learn any more than they have, are not induc'd by Affection to do for those under their care all the Good that they can. Since then the Affairs either of Men's Callings, or of their private Estates, or the Service of their Country (all which are indispensibly their Business) allows them not the leisure to look daily after the Education of their Children; and that, otherwise, also they are naturally less capable than Women of that Complaisance and Tenderness, which the right Instruction and Direction of that Age requires; and since Servants are so far from being fit to be rely'd upon in that great concern, that to watch against the Impediments they actually bring thereto, is no small part of the care that a wise Parent has to take; I do presume that (ordinarily speaking) this so necessary a Work of forming betimes the Minds of Children so as to dispose them to be hereafter Wise and Vertuous Men and Women, cannot be perform'd but by Mothers only. It being a thing practicable but by a very few to purchase the having always Wise, Vertuous and well Bred People, to take the place of a Parent in governing their Children; and together with them such Servants and Teachers, as must peculiarly be employ'd about them; For the World does not necessarily abound with such Persons as these, and in such circumstances as not to pretend to more profitable employments than Men of one or two thousand Pounds a Year (and much less those great numbers who have smaller Estates) can often afford to make the care of governing their Children from their Infancy to be. The procuring of such a Person as this may (by accident) sometimes be in such a ones Power; but to propose the ingaging for reward whenever there shall be need for them, vertuous, wife, and well-bred Men and Women, to spend their time in taking care of the Education of young Children, is what can be done but by a very few; since the doing this would not be found an easy charge to the greater part of almost any rank amongst us; unless they would be content for the sake hereof to abridge themselves of some of their extravagant Expences; which are usually the last that Men will deny themselves.(pp. 80-81)
You can find Locke's 'Treatise of Education' online. She considers this education to be particularly important in matters of religion, since she accepts the common view that some sort of basic rational religion (God, immortality of the soul, moral law) is, while perhaps not absolutely necessary to a moral society, is nevertheless especially conducive to it, when it is taken sufficiently seriously and is combined with a generally diffused education about important matters. Masham is very explicit that she is not just arguing that the women of her time should have a better education, which she thinks is obviously due to them as rational creatures; she is also trying to persuade men. She is not running a speculative argument, holding that in the abstract women should have a good education; she is advocating a practical program for the improvement of society, and wants to effect actual change:
But it is not perhaps very seasonable to propose that Ladies should have any greater Accomplishments or Improvements of their Understandings than the well discharging of their Duty requires, till it is thought fit for them to have that: The advantages of which to Men themselves, and the necessity thereof to a right Education of their Children of both Sexes are too evident, when reflected upon, not to obtain Encouragement of so much Knowledge in Women from all who are Lovers of Vertue, were it not true that Conviction does not always operate. The Law of Fashion or Custom, is still to be obey'd, let Reason contradict it ever so much: And those bold Adventurers are look'd upon but as a sort of _Don Quixots_; whose Zeal for any Reformation puts them upon Combating generally receiv'd Opinions, or Practices; even tho' the Honour of their Maker be concern'd therein: Or (what is nearer to most) their own Private and Temporal Interests. I am sure that a just consideration of both these furnishes every one with very cogent inducements to make what opposition they can to Immorality, both by amending their own faults, and by indeavouring to prevail upon others to correct whatever has contributed to the making us a vicious People.(pp. 98-99)
In a sense, Masham's work on education epitomizes the Enlightenment stereotype: education being key to the moral society, it is essential for us to have better and more widely available education in order to improve our morals. But the argument is in many ways unique, particularly given its well-thought-out focus on domestic education (which in some respects far exceeds Locke's more famous discussion in subtlety and reasoning); and it is a pity that the text did not have a wider audience. Indeed, it's a pity that it does not have a wider audience today. Fortunately, thanks to Project Gutenberg and ManyBooks.net, you can read it in a handy online edition.