… is from page 51 of the 2012 revised and updated edition of Steven Landsburg’s The Armchair Economist :
Policymakers need a dose of abstraction to keep their heads out of the clouds. It is easy to get carried away making long lists of pros and cons, all the while forgetting that sooner or later we must decide how many cons it takes to outweigh a particular pro. We can commission experts to estimate costs and benefits, but when the costs are measured in apples and the benefits in oranges, mere arithmetic can’t illuminate the path to righteousness. When all the facts are in, we still need a moral philosophy to guide our decisions.