21st Century Classical Greek -- "due and owing" nuance of impersonal gerundive

Book I section 21.

ἐκ δὲ τῶν εἰρημένων τεκμηρίων ὅμως τοιαῦτα ἄν τις νομίζων μάλιστα ἃ διῆλθον οὐχ ἁμαρτάνοι, καὶ οὔτε ὡς ποιηταὶ ὑμνήκασι περὶ αὐτῶν ἐπὶ τὸ μεῖζον κοσμοῦντες μᾶλλον πιστεύων, οὔτε ὡς λογογράφοι ξυνέθεσαν ἐπὶ τὸ προσαγωγότερον τῇ ἀκροάσει ἢ ἀληθέστερον, ὄντα ἀνεξέλεγκτα καὶ τὰ πολλὰ ὑπὸ χρόνου αὐτῶν ἀπίστως ἐπὶ τὸ μυθῶδες ἐκνενικηκότα, ηὑρῆσθαι δὲ ἡγησάμενος ἐκ τῶν ἐπιφανεστάτων σημείων ὡς παλαιὰ εἶναι ἀποχρώντως.

[2] καὶ ὁ πόλεμος οὗτος, καίπερ τῶν ἀνθρώπων ἐν ᾧ μὲν ἂν πολεμῶσι τὸν παρόντα αἰεὶ μέγιστον κρινόντων, παυσαμένων δὲ τὰ ἀρχαῖα μᾶλλον θαυμαζόντων, ἀπ᾽ αὐτῶν τῶν ἔργων σκοποῦσι δηλώσει ὅμως μείζων γεγενημένος αὐτῶν.

In subsection 1, heuristhai followed by de could be a substantive, because it is a base voice impersonal gerundive in perfective conceptual. Note that this cannot be reflexive as Middle Liddell wants you to think because that’s what they say “middle voice” is, but it possibly refers to a discovery to which ancient and obvious signs lead, and not something deliberately sought out.

Jowett wants you to think of it as making up one’s mind to come to a given conclusion; he is misled by the idea that this would be the reflexive middle voice.

The other possibility is something I mentioned before for the i.g. It means a sort of command to be kept because it is due and owing. In our context here, it would mean telling people that once they have studied the data sufficiently, they are bound to come to specific conclusions. This repeats the concept in the first part of the subsection, that those who take into account the grounds on which Thucydides bases his opinion, will not go wrong in agreeing with his conclusions.