My advice for how to interpret that difficult passage in Nic. Eth. II.6? Simply continue reading beyond the passage-- since then it becomes clear, I think, that the words which I place in brackets below are not meant to be part of the definition at all, but rather a comment upon the definition:
Ἔστιν ἄρα ἡ ἀρετὴ ἕξις προαιρετική, ἐν μεσότητι οὖσα τῇ πρὸς ἡμᾶς [ὡρισμένη λόγῳ καὶ ὡς ἂν ὁ φρόνιμος ὁρίσειεν].If we bracket those words for the moment, then the definition of virtue given here (which is said, btw, to state the genos only, at 1114b27), is of a trait (ἕξις) which disposes someone to choose in a certain way (προαιρετική), and which is intermediate (ἐν μεσότητι οὖσα) as between two other states, in a manner relative to us (τῇ πρὸς ἡμᾶς). As Taylor points out in his commentary, that and precisely that is what Aristotle had established in his discussion which precedes the definition.
But what is the reason for the bracketed words? They comment upon the definition, and Aristotle gives the sense of that comment in the lines that immediately follow.
First Aristotle clarifies his definition with a couple of glosses:
μεσότης δὲ δύο κακιῶν, τῆς μὲν καθ' ὑπερβολὴν τῆς δὲ κατ' ἔλλειψιν· καὶ ἔτι τῷ τὰς μὲν ἐλλείπειν τὰς δ' ὑπερβάλλειν τοῦ δέοντος ἔν τε τοῖς πάθεσι καὶ ἐν ταῖς πράξεσι, τὴν δ' ἀρετὴν τὸ μέσον καὶ εὑρίσκειν καὶ αἱρεῖσθαι.
It is a trait intermediate between two vices one of which is so by excess and the other by defect. And they are so because they either fall short of or exceed what they should, in emotions or in actions, whereas virtue identifies and chooses the intermediate mark.
The reason for this last gloss is that Aristotle is perfectly well aware that, to anyone who has actually striven to be virtuous, it will seem strange and even paradoxical to say that virtue is something intermediate. Rather, from the point of view of someone striving to be good (κατὰ δὲ τὸ ἄριστον καὶ τὸ εὖ), virtue looks like a nearly unattainable pinnacle. (Think, e.g. of the Congressional Medal of Honor, awarded it is said to "the bravest of the brave", for those who distinguish themselves "…conspicuously by gallantry and intrepidity at the risk of his life above and beyond the call of duty...". Nothing intermediate about that! And there are lots of other examples.)
So Aristotle thinks that he has to defend his definition. He therefore says that, in fact, as regards the nature of a virtue, and considering it formally (i.e. with a view to its logos), each virtue, it turns out, falls between two opposing vices. Someone immersed in the task of trying to act well might not notice this; yet this will be how a discerning person will define it.
And this is exactly what he had said in the bracketed words, and so the text should be translated:
Virtue is a trait (ἕξις) which disposes someone to choose in a certain way (προαιρετική), and which is intermediate (ἐν μεσότητι οὖσα) as between two other states, in a manner relative to us (τῇ πρὸς ἡμᾶς)-- that is, when virtue is marked out by its formal definition (ὡρισμένη λόγῳ ), and in the way that a person with insight into practical matters would mark it out (ὡς ἂν ὁ φρόνιμος ὁρίσειεν).This explains perfectly the reading of the codices -- the key is to see that the problematic words are not in the definition but about the definition--
-- and it also shows why all of those dozens of articles on the passage should quietly be pushed aside and forgotten.