ὅτι τὸ ψυχαγωγοῦν ἅμα καὶ τὴν ὠφέλειαν ἐπιφέρον τοῖς φιλομαθοῦσι τοῦτ᾽ ἔστιν ἡ τῶν αἰτιῶν θεωρία καὶ τοῦ βελτίονος ἐν ἑκάστοις αἵρεσις.
Translation (reversed for readability):
The theory of causes and of the best choice in each (case) is this that (is) the (thing) seducing and bringing help to the curious
Is there an implied copula (is) as I have indicated? Seems there has to be or else the subordinate lacks a verb.
But this could be said of the main sentence as well. There is no verb in either clause.
What connects them is ἔστιν with τοῦτ᾽ referring to the entire subordinate clause.
I would also read the subordinate as relative clause with ὅτι representing ὅ τι.
This is supported by the Perseus translation (thanks for the link, btw):
What is really educational and beneficial to students of history is the clear view of the causes
of events, and the consequent power of choosing the better policy in a particular case.
You lost me there. I could probably agree with everything you wrote except this bit. Isn’t ἔστιν the verb in the main clause? If not, then what is it?
I may be wrong. I’ve always looked at ἔστιν as a linking verb, that is, as neither
belongs to the subject nor the predicate.
Sure, but linking verbs are still verbs. Different grammarians define “predicate” differently. At least that seems to be my impression. I believe you about oti, but I can’t bear to think about “that” again. I had a thread about “that” not too long ago and it ran out of gas before yielding any great insight.