Economics is a young science, and there is still much that we do not know. Economists cannot be completely confident when they assess the effects of alternative policies. This ignorance suggests that economists should be cautious when offering policy advice.
This acknowledgement of the unknown unknowns reminds me of (aside from Donald Rumsfeld) Nassim Nicholas Taleb and our inability to foresee the impact of the highly improbable, or in some cases, the impact of the even the most probable. As a visual representation of accounting for what we don't know we don't know, Taleb proposes that we keep a bookshelf that includes plenty of books we've never read. This strategy, of course, implies that that untapped knowledge may still serve as a future resource. I'll use this as justification for the ambitious unread books collecting dust next to my bed.