… is from page 69 of Hayek’s 1991 collection, The Trend of Economic Thinking , which is Vol. 3 in The Collected Works of F.A. Hayek; specifically, it’s from Hayek’s 1954 essay “History and Politics”:
The complexity of social events in particular is such that, without the tools of analysis which a systematic theory provides, one is almost bound to misinterpret them; and those who eschew the conscious use of an explicit and tested logical argument usually merely become the victims of the popular beliefs of their time.
The economic way of thinking makes visible much that is invisible to the naked (that is, to the non-economically focused) eye. The economic way of thinking puts one on high alert against claims and suggestions – of which there are surprisingly many – that free lunches are widely available. And the economic way of thinking fosters a healthy distrust of anyone who, expressing fine intentions, pleads for power to order others about.