This quote is often attributed to the Latin philosopher Boethius of the late fifth and early sixth centuries. It translates literally as, "If you had been silent, you would have remained a philosopher." The phrase illustrates a common use of the subjunctive verb mood. Among other functions it expresses actions contrary to fact. Sir Humphrey Appleby translated it to the PM as: "If you'd kept your mouth shut we might have thought you were clever."
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