If there’s one thing we should have learned in the decade since 9/11, it’s that “mission accomplished” is far too definitive (and facile) a way to describe progress on a complex problem — be it a Middle East war, a financial crisis, or stubbornly high unemployment. The basis of strong leadership post-9/11 is the realization that complicated situations always appear simpler than they are and just when you think you’ve got them solved, they prove you wrong. How do you stay mentally flexible? The strongest leaders keep their thinking fluid by grounding it on this crucial assumption: reality is constructed by us to a larger extent than it is received by us. This starting point protects them from the tidy, slogan-styled solutions that 21st-century challenges resist.