… is from page 20 of Alec Cairncross’s March 1992 BNL Quarterly paper, “From Theory to Policy-Making: Economics as a Profession“:
When it comes to action, economic theory is only one input among many. It has to be combined with a grasp of political and administrative feasibility and above all has to take advantage of experience and observation, not rely wholly on logic. As has often been remarked, logic can be a way of going wrong with confidence.