ingenium par negotiis
From chapter 9 of the Tale of a Tub

Tacitus uses this phrase twice, in each case of men who, with certain qualities unfitting them for public business, yet proved adequate to its discharge. 'Equal, but not more than equal,' he says in one case. Swift evidently had only a vague recollection of the passages, as he uses the phrase of a talent which was specially adapted to public work.