Mr Darling ends the letter by quoting advice from economist Maynard Keynes that "when the facts change, you change your mind".

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That is you, careful and diligent reader that you are, and I, we are entirely happy to change our minds when the facts change.

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He was often exasperated by the inconsistencies in Keynes' work and his tendency to change his mind - something the Cambridge economist did quite regularly, and not only "when the facts changed".

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