‘Ours is not to reason why, ours is but to do or die’ is often heard, but is wrong. The lines from Tennyson’s ‘The Charge of the Light Brigade’ are ‘Their’s not to reason why,/ Their’s but to do and die’. Note that the closing words ‘do and die’ give the lines an entirely different sense. Finally, it should be noted that Tennyson’s punctuation of ‘theirs’ is irregular.
- Bill Bryson, Troublesome Words, 3rd ed. paperback, Penguin Books, 2001, p. 149.