Hi, a few points to note here:
A μέν … δέ pair need not have the same mood or construction in both clauses. The particles simply bring the two clauses into a certain relation, but each can have its own construction. To take just one example, look at the change of moods in the clauses with μέν and δέ in Plato’s Euthyphro 3d–e: no need for the two protases to keep the same mood; instead, each has the construction appropriate for its combination of protasis and apodosis:
εἰ > μὲν > οὖν, ὃ νυνδὴ ἔλεγον, > μέλλοιέν > μου καταγελᾶν ὥσπερ σὺ φῂς σαυτοῦ, οὐδὲν ἂν εἴη ἀηδὲς παίζοντας καὶ γελῶντας ἐν τῷ δικαστηρίῳ διαγαγεῖν· εἰ > δὲ > > σπουδάσονται> , τοῦτ᾽ ἤδη ὅπῃ ἀποβήσεται ἄδηλον πλὴν ὑμῖν τοῖς μάντεσιν.
On the sentence from Aristotle, I don’t see anything too unusual there. Aristotle is clearly showing off a little with a grand flourish concluding his study: look at how he rhetorically sets off pairs of the same words:
Καὶ ἡ μὲν > ἀρχὴ > τῆς > ἀρχῆς > εἴη ἄν, ἡ δὲ > πᾶσα > ὁμοίως ἔχει πρὸς τὸ > ἅπαν > πρᾶγμα.
I won’t enter the minefield of interpreting what’s going on here, however the Greek seems (perhaps deceptively?) straightforward. It just gets confusing because of the rhetorical flourish.
The final para runs (I’ll break it into three parts, and bold two clauses):
Ἐπεὶ δὲ τῶν περὶ τὴν διάνοιαν ἕξεων, αἷς ἀληθεύομεν, αἱ μὲν ἀεὶ ἀληθεῖς εἰσίν, αἱ δὲ ἐπιδέχονται τὸ ψεῦδος, οἷον δόξα καὶ λογισμός, ἀληθῆ δ᾿ ἀεὶ ἐπιστήμη καὶ νοῦς, καὶ οὐδὲν ἐπιστήμης ἀκριβέστερον ἄλλο γένος ἢ νοῦς, αἱ δ᾿ ἀρχαὶ τῶν ἀποδείξεων γνωριμώτεραι, ἐπιστήμη δ᾿ ἅπασα μετὰ λόγου ἐστί, τῶν ἀρχῶν ἐπιστήμη μὲν οὐκ ἂν εἴη, ἐπεὶ δ᾿ οὐδὲν ἀληθέστερον ἐνδέχεται εἶναι ἐπιστήμης ἢ νοῦν, > νοῦς ἂν εἴη τῶν ἀρχῶν> ,
ἔκ τε τούτων σκοποῦσι καὶ ὅτι ἀποδείξεως ἀρχὴ οὐκ ἀπόδειξις, ὥστ᾿ οὐδ᾿ ἐπιστήμης ἐπιστήμη. Εἰ οὖν μηδὲν ἄλλο παρ᾿ ἐπιστήμην γένος ἔχομεν ἀληθές, > νοῦς ἂν εἴη ἐπιστήμης ἀρχή> .
Καὶ ἡ μὲν ἀρχὴ τῆς ἀρχῆς εἴη ἄν, ἡ δὲ πᾶσα ὁμοίως ἔχει πρὸς τὸ ἅπαν πρᾶγμα.
Look at how Aristotle has used ἀρχή in two different ways set off against νοῦς.
- In the first one, he’s saying νοῦς would be of αἱ ἀρχαί (objective genitive).
- In the second, he says νοῦς itself would be ἀρχή (of ἐπιστήμη).
Mix those two together, and you get:
ἐπιστήμης ἀρχὴ [= νοῦς, according to the second cl. in bold] ἂν εἴη τῶν ἀρχῶν [from the first cl. in bold].
Putting the objective genitive in the singular to round out the flourish and you get the first part of the final sentence:
Καὶ ἡ μὲν ἀρχὴ [in the sense of the second cl. in bold] τῆς ἀρχῆς [in the sense of the first cl. in bold] εἴη ἄν [less usual positioning of the verb with particle in the sentence, but this would be, I guess, to rhetorically bring together the two forms of ἀρχή at the beginning].
The second part of the final sentence doesn’t need to be in the same mood as it’s not following from the reasoning earlier in the paragraph (he’s not talking about νοῦς anymore, but about ἐπιστήμη).
In other words, νοῦς (of αἱ ἀρχαί) is ἀρχή of ἐπιστήμη, and ἐπιστήμη stands in the same way to τὸ ἅπαν πρᾶγμα as νοῦς to αἱ ἀρχαί. This sentence summarises the chain from the ἀρχή to ἐπιστήμη of τὸ ἅπαν πρᾶγμα in a grand conclusion.
I haven’t read any commentary on this particular part, it’s just how the Greek seems to read (to me).
Cheers, Chad