To locate the exact phrase and understand how the grammar operates, we have to look closely at Plutarch's masterful blending of certainty and conditional hypothesis.

1. The Greek Text for "But one single thing is certain"

In the original text of Life of Antony 59.4, Geminius does not use an abstract philosophical word for "certainty." Instead, he links his own sobriety/drunkenness to absolute knowledge. The exact Greek text for that transition is:

"...ἓν δ᾽ εἰδέναι καὶ μεθύων καὶ νήφων..."

Literal Breakdown:

• ἕν δέ: "but one thing"
• εἰδέναι: "to know" (an infinitive reporting his direct statement: "he said that he knew...")
• καὶ μεθύων καὶ νήφων: "both while drinking/drunk and while sober" [1]

When combined with the rest of his retort, the entire sentence reads:

ἀπεκρίνατο, τὰ μὲν ἄλλα νήφοντος εἶναι λόγου, ἓν δ᾽ εἰδέναι καὶ μεθύων καὶ νήφων, ὅτι βέλτιον ἂν ἔχοι τὰ πράγματα Κλεοπάτρας εἰς Αἴγυπτον ἀποσταλείσης.
(He answered that the remaining matter required a sober conversation, but one thing he knew, whether drunk or sober: that things would be better if Cleopatra were sent away to Egypt.)

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2. Reconciling the "Certainty" with the "Hypothetical Optative"

Your stylistic instinct is sharp: how can Geminius claim to know something as an absolute certainty, only to immediately frame it in a hypothetical, conditional optative form (βέλτιον ἂν ἔχοι / ἂν ἔχειν)?

This is not a grammatical contradiction; it is a deliberate rhetorical device found in both classical Greek and Latin.

In Greek: The Certainty of a Logical Law

Geminius's "certainty" applies to the cause-and-effect relationship itself, not to whether the condition will actually happen. [1]

• He is absolutely sure of a political law: Condition A (Cleopatra leaves) will strictly result in Outcome B (Affairs improve).
• The use of the potential optative with ἄν (βέλτιον ἂν ἔχοι) paired with the genitive absolute(Κλεοπάτρας... ἀποσταλείσης) is a standard future less vivid condition ("If X should happen, Y wouldfollow").
• By using the optative, Geminius remains a diplomatic guest: he treats Cleopatra's departure as a polite, hypothetical suggestion ("should she be sent away") rather than aggressively demanding it as an imperative command to Antony's face. His "certainty" is that this logical formula is foolproof. [1]

In Latin: Reconciling into Oratio Recta

When converting Geminius's quote into direct speech (oratio recta) in Latin, you can beautifully preserve this exact tension between absolute certainty (scio) and the conditional future reality by using a mixed conditionor a future less vivid subjunctive construction. [1]

Idiomatic Latin Reconstruction:

"Unum illud vel ebrius vel sobrius certe scio: melius se habeant res, si Cleopatra in Aegyptum remittatur."
(This one thing, whether drunk or sober, I know for certain: your affairs would fare better, if Cleopatra should be sent back to Egypt.)

Why this works grammatically in Latin:

1. The Certainty: Unum illud... certe scio ("This one thing I know for certain") uses a strong present indicative. It mirrors ἓν δ᾽ εἰδέναι. Geminius is stating an absolute fact about his own mind.
2. The Conditional: melius se habeant res, si... remittatur is a Present Subjunctive Condition (Future Less Vivid / "Should-Would"). Just like the Greek optative, the subjunctive here shifts the focus. It implies: "I am 100% certain about the mathematical certainty of this outcome, even though her departure itself remains a hypothetical possibility."

If you are looking to refine this text further, would you like to see how this banquet scene reads in Sir Thomas North's famous 1579 English translation—the very version Shakespeare used to write Antony and Cleopatra?