... entity in the Principles was the following : Whatever may be an object of thought , or may occur in any true or false proposition , or can be counted as one , I call a term ... mathematics unworkable . " For he had written in the Principles ... This volume of Bertrand Russell's Collected Papers finds Russell focused on writing Principia Mathematica during 1905–08. Eight previously unpublished papers shed light on his different versions of a substitutional theory of logic, with its elimination of classes and relations, during 1905-06. A recurring issue for him was whether a type hierarchy had to be part of a substitutional theory. In mid-1907 he began writing up the final version of Principia, now using a ramified theory of types, and eleven unpublished drafts from 1907-08 deal with this. Numerous letters show his thoughts on the process. The volume's 80-page introduction covers the evolution of his logic from 1896 until 1909, when volume I of Principia went to the printer.