It is in "August Comte and Positivism". Stuart Mill states that "the only security against this narrowness is a liberal mental cultivation, and all it proves is that a person is not likely to be a good political economist who is nothing else." (_The Collected Works of John Stuart Mill_, volume 10, page 306, which is avaiable at http://olldownload.libertyfund.org/Texts/MillJS0172/Works/Vol10/0223.10.pdf). Hugo E. A. da Gama Cerqueira