Some English usages are so common and so simple that one barely notices them; and yet are curious, and lead one into curious byways. Why, for example, might I write This is my laptop and yet This laptop is mine? What distinguishes my from mine? In meaning, nothing whatever; they both mean of me or belonging to me. So why do we have these two slightly different words a difference replicated in thy and thine, her and hers, our and ours, and so on?
Both words began as the Old English min. As many words do in any language, in speech it often dropped the final sound, the -n, to become my. The old form survived, today spelled mine. And now we use the two in different ways. But why? The French language makes a like distinction between mon and mien. German, using mein for both purposes, makes none. Nor did Latin. And the same goes for similar words in those languages: we and the French are on one side, German and Latin on the other. Yet Germany (via Anglo-Saxon) and Rome were the main links between English and its ancient origins in India. We must find the reason elsewhere.
The distinction has arisen gradually. The great 1611 translation of the Bible always uses mine where we today would do so. But no doubt for euphony it also uses mine before nouns beginning with a vowel, where we would put my, as in mine eyes and mine enemies, for frequent example. Ben Jonson, writing at much that time, began his most famous poem, Drink to me only with thine eyes. But those usages were already antique, and were soon dead, except in the deliberately archaic or mock-biblical English of, for instance, Victorian hymn-writers, as in Americas Mine eyes have seen the glory of the coming of the Lord. Today the distinction between my and mine, and similarly with the other personal possessives like them, is clear-cut.
That makes it all the more curious. We dont need two words, but we have them. And a look at their siblings her/hers, our/ours and so on makes it odder still. You can imagine my and the euphonious mine developing as strict parallels and then growing slowly apart for todays rival uses. But there was no need to add an -s to make the other words better-sounding before a vowel, and in fact it didnt happen: no one ever wrote yours eyes or theirs enemies. Ive never met any persuasive explanation of why double forms exist.
Theres a further oddity here. The 1611 Bible spelt all these words just as we do today: hers, ours and so on. But later they developed an apostrophe: Jane Austen (picture) habitually wrote hers and ours. As late as the 1850s Tennyson was writing Theirs not to reason why. Again, I dont know why. Would the 17th-century biblical translators have used an apostrophe, if it had been common during their day, which it wasnt? We cant know. But that doesnt help to explain why it arose in the Austenish hers and ours was she somehow thinking of this as the genitive -s? nor why it vanished again. But certainly it has. A late editor of mine once wrote theirs and was rightly mocked by his staff, even when he called Tennyson to his aid.
And there we open a new can of worms. An editor of mine? Why do we need that of? We do (though you can write poetically O mistress mine), yet mine already means of me. Why, in effect, put of twice over? Theres a parallel in phrases like a cousin of his wifes just as good English, though a touch colloquial, as of his wife. How come? Mine not to reason why.