theirs not to reason why

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theirs (or ours) not to reason why

it is not someone's place to question a situation, order, or system.
This phrase comes from Tennyson's poem ‘The Charge of the Light Brigade’ ( 1854 ), which describes how, in a notorious incident in the Crimean War, the British cavalry unquestioningly obeyed a suicidal order to ride straight at the Russian guns.
See also: not, reason, theirs, why