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;
Quote“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.
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And here is Peter Saint-Andre's text and translation at Monadnock;
Quote68. 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;
QuoteThe 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.
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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.