placed in the body like a rough diamond; and must be polished, or the lustre of it will never appear. And ’tis manifest, that as the rational soul distinguishes us from brutes; so education carries on the distinction, and makes some less brutish than others. 

He compares each individual's soul to "a rough diamond." It has great potential, but it "must be polished" so that its potential can be realized. He then proceeds to say that "the rational soul" is what makes humans different and superior to "brutes." Therefore, women should be educated so as to make them achieve their potential and their rightful places as above "the brutes."

Defoe goes on to appeal to the reader's sense of reason, as well as his ego, by saying that a rational man should want his companion to be educated and a proper companion to him. He writes,

Not that I am for exalting the female government in the least: but, in short, I would have men take women for companions, and educate them to be fit for it. A woman of sense and breeding will scorn as much to encroach upon the prerogative of man, as a man of sense will scorn to oppress the weakness of the woman. 

Basically, it is in men's best interest that women be educated. Defoe italicizes the key line for emphasis, as well.

About midway through the essay, Defoe uses a listing technique to highlight some of his important points. This list warns of the negative results of not educating women. The first few lines read:

If her temper be good, want of education makes her soft and easy.

Her wit, for want of teaching, makes her impertinent and talkative.

Her knowledge, for want of judgement and experience, makes her fanciful and whimsical.

Women have certain traits and capabilities, but without education, those traits can become negative rather than positive. Defoe uses this listing technique, along with a pertinent simile and appeals to religion, ego, and common sense, to illustrate the benefits of educating women. 

There are, of course, many rhetorical devices in this short essay.  I will list a few of them:

• Defoe says that the soul is placed in the body "like a rough diamond" and that, like the diamond, it needs to be polished.  The soul, he says, needs to be polished by education.  This is a simile.
• He asks "But why then should women be denied the benefit of instruction?"  This is, of course, a rhetorical question.  There are quite a few of these in the same paragraph.
• For one more, here is an example of hyperbole

And, without partiality, a woman of sense and manners is the finest and most delicate part of God's Creation, the glory of Her Maker, and the great instance of His singular regard to man, His darling creature: to whom He gave the best gift either God could bestow or man receive. And 'tis the sordidest piece of folly and ingratitude in the world, to withhold from the sex the due lustre which the advantages of education gives to the natural beauty of their minds.

This passage seems to be going to excess in praise of a woman of sense and manners.