In Cic. de Fin. 5.83:
"in virtute enim sola et in ipso honesto cum sit bonum positum, cumque nec virtus, ut placet illiis, nec honestum crescat, idque bonum solum sit, quo qui potiatur, necesse est beatus sit,".
The translation (Lacus Curtius):
For since the good consists solely in virtue and in actual Moral Worth, and neither virtue nor Moral Worth, as they hold, admits of increase, and since that alone is good which necessarily makes its possessor happy,".
From ("cum" = since, plus subjunctive) "...idque bonum solum sit, quo qui potiatur, necesse est beatus sit," the English translation clearly works but it leaves some questions. The literal translation:
"and since that ('the increase') alone is good, from which (quo) it is necessary (necesse est) that he who possesses this (qui potiatur) is happy (beatus sit),";
this implies that "to be happy" is necessary--an obligation--which happiness isn't.
(i) The adverb "necessarily" is "necessario"; for "necesse est" Oxford lists one meaning, "it is necessary"--nothing else. If this sentence only works with adverb, "necessarily", why didn't Cicero use "necessario"?
(Interesting is this from Caes. de Bel. Gal. 7.19.4:
"quanto detrimento et quot virorum fortium morte necesse sit constare victoriam;" =
"with how great loss and the death of how many gallant men the victory would necessarily be purchased;"'
The present subjunctive "necesse sit" = "would be necessary" might have been better than "necesse est", in Cicero's example: "cum" = "since" was already dictating the use of the present subjunctive (sequence-of-tenses)).
(ii) Then, "which necessarily makes its possessor happy"; there is no verb, in the Latin, for something making anything happen.
(iii) Deponent verb, "potior", selects the ablative, sometimes the genitive, according to context. Does this explain why the first relative pronoun, "quo", is in the ablative; or, would "quo" have been used anyway, because it fits?