The two bolded phrases seem to me to be contradictory: in the first Thucydides seems to be stating that he is not incorporating into his writing evidence gathered “from the person who happened to be present” or eye-witness accounts; in the second he seems to be stating that he is incorporating such accounts into his history as an auxiliary to his own observations of the war. The second seems to make much more sense; am I misinterpreting οὐκ ἐκ τοῦ παρατυχόντος πυνθανόμενος?
Feeling as much at a loss as you, I checked the Penguin translation. τοῦ παρατυχόντος seems to have the sense of ‘the first person I happen upon by chance’. Morris’s commentary in Perseus notes: “the first chance comer. This verb is used of persons or things which present themselves by accident.”
So, it seems like he’s contrasting not the source of his information, but the quality: ἐκ τοῦ παρατυχόντος πυνθανόμενος and ὡς ἐμοὶ ἐδόκει versus ὅσον δυνατὸν ἀκριβείᾳ
That’s right, it’s a fairly common sense of παρατυγχάνω. LSJ says,
often in partic. παρατυχών, > whoever chanced to be by, > i.e. > the first comer, any chance person> , Id.; – so, τὸ παρατυγχάνον or παρατυχόν > whatever turns up or chances> …
In colloquial English, ὁ παρατυχών is like, “any random person.”
I “third” the above comments. Take also into consideration the sentence that follows and is, for some reason, one of my favourites : " [1.22.3]ἐπιπόνως δὲ ηὑρίσκετο, διότι οἱ
παρόντες τοῖς ἔργοις ἑκάστοις οὐ ταὐτὰ περὶ τῶν αὐτῶν
ἔλεγον, ἀλλ’ ὡς ἑκατέρων τις εὐνοίας ἢ μνήμης ἔχοι. "
It further clarifies, as I see it, the difference between taking into consideration what the first person he chanced upon told him and doing his own research (not even trusting what eye-witnesses and participants told him just like that).
Ah, that makes much more sense. I was mistakenly under the impression that τοῦ παρατυχόντος was referring to someone who was present at the events into which Thucydides happened to be inquiring, not the random passerby.
Are you sure that “οις” and “περί των άλλων” stand in masculine gender? It seems to me that they are neutral pronouns, refering to “έργα”.
EDIT: Mea culpa! “των άλλων” is masculine…
In my opinion “παρά των άλλων” means “from other person who were present in the war acts”. You can see that by checking the parataxis τε… καί between “οις τε αυτός παρην” and “καί παρά των άλλων”. You can add “παρά των άλλων (των παρόντων)” in order to comprehend the differnce between “του παρατυχόντος” and “των άλλων”.
There seems to be a scholarly debate over the structure of this passage according to Marchant:
οἷς τε αὐτὸς παρῆν–i.e. (ἠξίωσα γράφειν ἐκεῖνἀοἷς: I described facts which I myself witnessed and (I described facts) after inquiring about each detail from others, thus ἐπεξελθών is parallel to οἷς αὐτὸς παρῆν, which, as Mr. Forbes says, is equivalent to αὐτὸς παραγενόμενος. (Another way is to make ἐπεξελθών govern οἶς αὐτὸς παρῆν, but this has the great disadvantage of making οἷς αὐτὸς παρῆν and παρὰ τῶν ἄλλων parallel phrases. See crit. note.)
Do not be confusud at all! “ἔργα” here does not mean “acts”, it means “battles, war acts”. That’s why he sais “ἔργα τῶν πραχθέντων…”= the battles among things done… See also 1.23 "τῶν δὲ πρότερον ἔργων μέγιστον ἐπράχθη τὸ Μηδικόν… "
The contrast is between “πυνθανόμενος” and “ἐπεξελθών” (see “ούκ… ἀλλά…”). There is no contradiction in “έκ τοῦ παρατυχόντος” and “παρὰ τῶν ἄλλων”. The meaning is
… to write down the battles not simply by inquiring imformation from any random person, and not from my point of view, but after examining (care)fully whatever (battle) I myself experienced and after examining (care)fully in every possible detail whatever information I got about each one (battle) from other (present) people…
Among the things done during the war, I thought worthy to write down the battles not by inquiring information from any random person, and not (also) from my point of view, but after examining carefully those battles, in which I myseslf was present, and (after examining carefully) about each one (battle) in every possible detail from other (present) people’s information.
There is some kind of “brachylogy”, if I am not wrong, in the sentence above.
And if you combine these with the next passage, as cited from IreneY, I think that you can understand fully what Thucydides wanted to say about examining the battles of the Peloponnesian war…
EDIT: The initial parenthesis was missing before “ἐπεξελθών ταῦτα”…
My sentiments exactly. The variatio exhibited by οἷς τε αὐτὸς παρῆν καὶ παρὰ τῶν ἄλλων seemed to me to be paradigmatic of Thucydidean prose. I don’t know why Marchant has a problem with “making οἷς αὐτὸς παρῆν and παρὰ τῶν ἄλλων parallel phrases.”
the above comments. Take also into consideration the sentence that follows and is, for some reason, one of my favourites 
: " [1.22.3]ἐπιπόνως δὲ ηὑρίσκετο, διότι οἱ