Note AG — BOOK I MORAL GOODNESS (continued 3)
Walter Miller translation
Cicero is guilty of a curious fallacy. If it follows from his premises, (1) some one virtue is the highest virtue, and (2) the duties derived from the highest virtue are the highest duties, and if (3) wisdom is the highest virtue, then it can only follow that the duties derived from wisdom are the highest duties. But Cicero throws in a fourth premise that the "bonds of union between gods and men and the relations of man to man" are derived from wisdom, and therewith sidetracks wisdom and gives the duties derived from the social instinct the place from which wisdom has been shunted.
Cicero could not refrain from introducing a bit of theoretical speculation that has no value for his practical position—it actually prejudices it and confuses the reader.