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Full Idea
Prescriptivists claim that there are rules of reasoning which govern non-descriptive as well as descriptive speech acts. The standard example is possible logical inconsistency between contradictory prescriptions.
Gist of Idea
If there can be contradictory prescriptions, then reasoning must be involved
Source
Richard M. Hare (Universal Prescriptivism [1991], p.455)
Book Ref
'A Companion to Ethics', ed/tr. Singer,Peter [Blackwell 1993], p.455
A Reaction
The example doesn't seem very good. Inconsistency can appear in any area of thought, but that isn't enough to infer full 'rules of reasoning'. I could desire two incompatible crazy things.