Cicero, de Oratore (2.25.108):
"...in quibus hoc praecipit ratio et doctrina ut vis eius rei, quam definias sic exprimatur ut neque absit quicquam neque supersit,"
"...on which occasions reason and learning direct, that the whole force of the thing which you define should be expressed in such a manner, that there be nothing omitted or superfluous;" (Attalus).
"...in quibus hoc praecipit ratio et doctrina..."
might be better translated as:
"...on which occasions reason and learning direct this...";
assuming that "hoc" = "this" is the neuter, accusative singular form. (The English translation omitted to translate "hoc".)
If (nominative, feminine nouns) "ratio" and "doctrina" are working together, as it were, in equal measure, to do the directing, why is the third-person singular, "praecipit", used and not the expected third-person plural, "praecipiunt"--"they (both) are directing (this)"? Does one noun take priority over another?