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What explanation can be put forward to describe why ἠκαιρεῖσθε is in the middle voice here?
[The antonym εὐκαιρεῖν is used in the active voice, both absolutely ἐλεύσεται δὲ ὅταν εὐκαιρήσῃ. (1 Corinthians 16:12), and in conjunction with an infinitive οὐδὲ φαγεῖν εὐκαίρουν. (Mark 6:31).]
Why ἠκαιρεῖσθε is middle is not the only thing that I find obscure here. Here are the others;
Ποτέ seems to being used of the (then) present time, rather than a time contrasted with the (then) present. It is logically more closely related to the explanation ἐφ’ ᾧ καὶ ἐφρονεῖτε "the attitude towards me,which you (at one time) used to hold".
That also sort of depends on whether ἤδη signifies that ἀνεθάλετε is to be thought of as occuring in the past and already finished or it is to thought of as in the past, but directly related to the present.
The metaphorical force of θάλλειν is, like any other metaphorical force, culturally dependant and therefore a priori obscure, perhaps related to the emergence of new growth in spring, or perhaps it is so well established in the metaphorical usage that it no longer employs the imagery of growth in plants at all. The paucity of derivative forms such as this ἀνα-θάλλειν suggests that the preposition here (and in other places where the preposition is used) is context dependent (part of the construction of meaning into the sentence he was writing, rather than that the preposition attached to θάλλειν carries a meaning into situations independent of the construction of the current sentence. [I hope that's not difficult to understand, but if it is, then contrast ἀναβοᾶν "to cry out loudly", where the ἀνα- describes something about how the crying out is done that can be used in any situation. A word like ἀναδοῦναι used in the sense of "to give on to somebody else something that I was given", "to deliver" seems more dependent on the story being told, than on the meaning the word itself carry in any number of situations, like when it is used in a description of something given upwards - roughly equivalent to an adverb of maner. Another well-known example is whether ἀνίστημι carries the force of "stand like I was before", or "stand with the body up as contrasted to (sitting or lying) down". For συν- the difference is between, "in coming together in this instance, we are able to ... " (the base meaning of the verb is meant - and the prefix relates it to the syntax), and "this is done together". For ἐπι- it is often useful to imagine the prefixed ἐπι- could be rendered with τοῦτο or ὅ to test whether it could indicate some dependency of the preceding thought in the text as a syntactically dependent prefixed preposition. The slightly more complex style of Luke-Acts and some of the epistles display this feature more often than the plainer narratives.]
It is also not clear to me the extent that the ὑπέρ further nominalises the infinitive, or relates to it adverbally.
The relationship (perhaps balance - preempting what will be said) between the ἤδη, the ἀνα- and the καὶ (ἐφρονεῖτε) is either masterful of a little confused.
The relationship (and sequence) of past events to the present is logical enough, but there expression is not so straightforward.
What explanation can be put forward to describe why ἠκαιρεῖσθε is in the middle voice here?
[The antonym εὐκαιρεῖν is used in the active voice, both absolutely ἐλεύσεται δὲ ὅταν εὐκαιρήσῃ. (1 Corinthians 16:12), and in conjunction with an infinitive οὐδὲ φαγεῖν εὐκαίρουν. (Mark 6:31).]
I've noted Mike's comment and the further elaboration Stephen has offered. I think Mke is right here to say we'd have a better notion if we had more instances of the verb's usage, but DGE (see Logeion) offers additional support for middle-passive usage;
it's also the case that we don't have much doubt about what Paul is saying in this rather informally-phrased locution: "Your impulsive thoughtfulness on my behalf has deeply gratified me -- the fact that you wanted to do something but had no opportunity." It seems to me that ἠκαιρεῖσθε here is a personal usage involving deprivation: Subject-affectedness is discerned and expressed in the middle voice here.
οὔτοι ἀπ’ ἀρχῆς πάντα θεοὶ θνητοῖς ὑπέδειξαν,
ἀλλὰ χρόνῳ ζητέοντες ἐφευρίσκουσιν ἄμεινον. (Xenophanes, Fragment 16)
Carl W. Conrad
Department of Classics, Washington University (Retired)