Who are capable of figuring the problem out

If I recall correctly DeWitt thinks that this was a direct jibe at Plato, who held that you have to be able to know geometry in order to be a philosopher. I'll look for a cite for that.

Hor., Sat. 1.3.66 = Horace's 'Satires' Book I, Satire III, line 66
Relevant fragment with line 66 highlighted:

[...]
But WE put virtue down to vice's score,
And foul the vessel that was clean before:
See, here's a modest man, who ranks too low
In his own judgment; him we nickname slow:
Another, ever on his guard, takes care
No enemy shall catch him unaware,
(Small wonder, truly, in a world like this,
Beset with dogs that growl and snakes that hiss);
We turn his merit to a fault, and style
His prudence mere disguise, his caution guile.
Or take some honest soul, who, full of glee,
Breaks on a patron's solitude, like me,
Finds his Maecenas book in hand or dumb,
And pokes him with remarks, the first that come;
We cry "He lacks e'en common tact." Alas!
What hasty laws against ourselves we pass!
For none is born without his faults: the best
But bears a lighter wallet than the rest.
A man of genial nature, as is fair,
My virtues with my vices will compare,
And, as with good or bad he fills the scale,
Lean to the better side, should that prevail:
So, when he seeks my friendship, I will trim
The wavering balance in my turn for him.
He that has fears his blotches may offend
Speaks gently of the pimples of his friend:
For reciprocity exacts her dues,
And they that need excuse must needs excuse.
[...]

Sen., Ep. 105.4 = Seneca's 'Moral letters to Lucilius' Letter 105, fragment 4:

[...]
As to not being feared, a moderate fortune and an easy disposition will guarantee you that; men should know that you are the sort of person who can be offended without danger; and your reconciliation should be easy and sure. Moreover, it is as troublesome to be feared at home as abroad; it is as bad to be feared by a slave as by a gentleman. For every one has strength enough to do you some harm. Besides, he who is feared, fears also; no one has been able to arouse terror and live in peace of mind.
[...]

Patrikios , the reference there is to Usener fragment U68, quoted here from Attalus.

Quote

[ U68 ]

Plutarch, That Epicurus actually makes a pleasant life impossible, 4, p. 1089D:

It is this, I believe, that has driven them, seeing for themselves the absurdities to which they were reduced, to take refuge in the "painlessness" and the "stable condition of the flesh," supposing that the pleasurable life is found in thinking of this state as about to occur in people or as being achieved; for the "stable and settled condition of the flesh," and the "trustworthy expectation" of this condition contain, they say, the highest and the most assured delight for men who are able to reflect. Now to begin with, observe their conduct here, how they keep decanting this "pleasure" or "painlessness" or "stable condition" of theirs back and forth, from body to mind and then once more from mind to body.

Aulus Gellius, Attic Nights, IX.5.2:

Epicurus makes pleasure the highest good but defines it as sarkos eustathes katastema, or "a well-balanced condition of the body."

That Epicurus Actually Makes a Pleasant Life Impossible is part of Against Colotes (Adversus Colotem), which in turn is bundled up in a massive collection of Plutarch's works called Moralia. The Internet archive has the Loeb set of Moralia that runs to 16 volumes in modern print. This is from Volume 14;

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“It is this, I believe, that has driven them, seeing for themselves the absurdities to which they were reduced, to take refuge in the 'painlessness' and the 'stable condition of the flesh,' supposing that the pleasurable life is found in thinking of this state as about to occur in people or as being achieved; for the 'stable and settled condition of the flesh' and the 'trustworthy expectation' of this condition contain, they say, the highest and the most assured delight for men who are able to reflect. (5.) Now first observe their conduct here, how they keep decanting this 'pleasure' or 'painlessness' or 'stable condition' of theirs back and forth, from body to mind and then once more from mind to body, compelled, since pleasure is not retained in the mind but leaks and slips away, to attach it to its source, shoring up 'the pleasure of the body with the delight of the soul,' as Epicurus puts it, but in the end passing once more by anticipation from the delight to the pleasure.

***

And here is Peter Saint-Andre's text and translation at Monadnock;

Quote

68. To those who are able to reason it out, the highest and surest joy is found in the stable health of the body and a firm confidence in keeping it.

τὸ γὰρ εὐσταθὲς σαρκὸς κατάστημα καὶ τὸ περὶ ταύτης πιστὸν ἔλπισμα τὴν ἀκροτάτην χαρὰν καὶ βεβαιοτάτην ἔχει τοῖς ἐπιλογίζεσθαι δυναμένοις.

δυναμένοις refers to capability, and ἐπιλογίζεσθαι (a word that also appears in the Principle Doctrine 22 and Vatican saying 35) seems to carry a meaning like 'reasoning it out'. This latter term might be an Epicurean neologism, and would possibly be a hapax if his works weren't frequently cited by friends and his critics alike.

So, 'those who are capable of reasoning/realizing/recognizing/figuring'...etc.

Cassius is correct that Dewitt thinks this is a jab at Plato, Timaeus 40d;

Quote

The words "those who are capable of figuring the problem out" are a parody of Plato's Timaeus 40d, where the text reads "those who are incapable of making the calculations" and the reference is to mathematical calculations of the movements of the celestial bodies, which "bring fears and portents of future events" to the ignorant. Baiting the adversary was a favorite sport of Epicurus.

And here is Timaeus 40d;

Quote

[40d] send upon men unable to calculate alarming portents of the things which shall come to pass hereafter,—to describe all this without an inspection of models1 of these movements would be labor in vain. Wherefore, let this account suffice us, and let our discourse concerning the nature of the visible and generated gods have an end.

***

μετὰ ταῦτα γενησομένων τοῖς οὐ δυναμένοις λογίζεσθαι πέμπουσιν, τὸ λέγειν ἄνευ δι᾽ ὄψεως τούτων αὖ τῶν μιμημάτων μάταιος ἂν εἴη πόνος: ἀλλὰ ταῦτά τε ἱκανῶς ἡμῖν ταύτῃ καὶ τὰ περὶ θεῶν ὁρατῶν καὶ γεννητῶν εἰρημένα φύσεως ἐχέτω τέλος.

Oh, and by the way Patrikios , I think the actual footnote is this;

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So I don't think those passages that TauPhi pulled for us are relevant to the question.

Edit; I see what happened. In chapter 14 the footnote is 99, not 88;

So the text in question is Plutarch, Non posse suaviter vivi secundum Epicurum. In other words, Epicurus Actually Makes the Pleasant Life Impossible.