Plutarch De E apud Delphos 5

[5] For it is, as the Delphians assume, - and on this occasion Nicander, the priest, spoke for them and said, the figure and form of the consultation of the god, and it holds the first place in every question of those who consult the oracle and inquire if they shall be victorious, if they shall marry, if it is to their advantage to sail the sea, if to take to farming, if to go abroad. ¹ But the god in his wisdom bade a long farewell to the logicians who think that nothing real comes out of the particle if combined with what the consultant thinks proper to undertake, for the god conceives of all the inquiries subjoined to this as real things and welcomes them as such. And since to inquire from him as from a prophet is our individual prerogative, but to pray to him as to a god is common to all, they think that the particle contains an optative force no less than an interrogative. If only I could, is the regular expression of a wish, and Archilochus ² says,

If to me it might be granted Neobulê’s hand to touch.

And in using if only they assert that the second word is added unnecessarily, like Sophron’s ³ surely:

Surely in want of children as well.

This is found also in Homer ⁴

Since I surely shall break your might

but, as they assert, the optative force is adequately indicated by the if.

[5] For the syllable EI ( if) is, as the Delphians conceive it, and as Nicander the priest (who was then present) also said, a conveyance and form of prayer to the God, and has the leading place in the questions of those who at every turn use it, asking if they shall overcome, if they shall marry, if it is convenient to go to sea, if to till the ground, if to travel. And the wise God, bidding adieu to the logicians, who think nothing at all can be made of this particle EI and any clause following it, understands and admits all interrogations annexed to it, as real things. Now, because it is proper for us to consult him as a prophet, and common to pray to him as a God, they suppose that this word has no less a precatory than an interrogatory power. For every one who prays or wishes says, εἰ γὰρ ὤφελον, O if I were, c. And Archilochus has also this expression: And they say that the second syllable in the word εἴθε is redundant like θήν in this of Sophron, Ἅμα τέκνων θὴν δευομένα, desiring also children; and in this of Homer, Ὡς θὴν καὶ σὸν ἐγὼ λύσω μένος, as I will also foil thy strength; but in the word EI there is sufficiently declared an optative power.