Forms of res are found three times in eight lines in Horace’s second epistle: rebus in 50, res (singular) in 51, rebus again in 57. This seems excessive, and the last instance is dubious in itself.(1) The context is clear enough, one of the paraenetic commonplaces that make up the bulk of the letter (56-59a):(2)
semper auarus eget: certum uoto pete finem.
inuidus alterius macrescit rebus opimis;
inuidia Siculi non inuenere tyranni
I suggest that rebus in 57 is an error facilitated by the previous rebuses (there is another in 36), and that Horace in fact wrote bobus, for three reasons:
1. It is more pointed. ‘Fat things’ or ‘fat possessions’ is terribly vague. Now the envious man grows thin seeing the fat cattle of his neighbor. This is far more vivid, though not quite so vividly specific as the parallel in the first Satire, where the greedy man is envious because someone else’s she-goat has a more distended udder (aliena capella gerat distentius uber, S. 1.110).
2. The following context fits better with bobus. Glossing Siculi . . . tyranni (58), commentators always mention Phalaris, often adding Agathocles and one (Dilke) or both (Wilkins) of the Dionysii. As tyrants, they all no doubt used ingenious tortures, but Phalaris’ brazen bull is by far the most famous of tyrannical Sicilian tortures.(3) As Mayer puts it: “Sicilian tyrants were bywords for cruelty; Phalaris of Agrigentum, for example, roasted people in a brazen bull.” With bobus, Phalaris is not just a parallel, a ‘for instance’, but an intertext. Horace’s maius tormentum now clearly alludes to Phalaris’ bull: he is saying that being seized by envy of another’s fat cattle is worse than being roasted in a brazen bull. The hyperbole is implausible, but Horatian.
3. The preceding context also fits bobus better. The primary meaning of certum . . . finem is abstract and ethical: ‘fixed moral limit’. As Mayer puts it, “The limit is, of course, quod satis est (46)”. However, looking back from 57-59a, certum . . . finem also suggests a visible illustration of the abstract moral limit and its violation: a farmer looking enviously at the greener grass and fatter cows in his neighbor’s pasture on the other side of a property-line (finem) clearly marked (certum) by a fence.
(1) The proximity of rebus 50 and res 51 seems even more suspicious: I can detect no point in the repetition. However, I see no way to remove either form.
(2) Shackleton Bailey quotes no variants, Klingner only ‘marcescit dett.’ in 57, which would destroy the rhetorical antithesis. Commentators mentioned by name include A. S. Wilkins (Macmillan, 1926), O. A. W. Dilke (London, 1954), and R. Mayer (Cambridge, 1994).
(3) I can’t think of another: can you, without consulting your books or the internet?