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