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