This innovative monograph is of major significance for not only students and academics undertaking research on the history of Mexico during the long dictatorship of Porfirio Díaz, but also scholars specializing in the history of ideas, philosophy and science. Unlike previous discussions of positivism in Latin America, this book presents a detailed analysis of the English thinker, Herbert Spencer’s original works as a necessary gateway into the discussion of the thinking of 'The Scientists'. Its principal purpose is to revisit the influential thesis of Leopoldo Zea which proposed that 'The Scientists' throughout this period were Spencerian positivists.