One major point of circular reasoning with Protestant thought on the identification of the canon is the concept of the canon as its own standard.  For example, the Reformers claimed that the New Testament books were obviously canonical because of their apostolic character.  But according to them where do we learn of the apostolic faith?  The canonical New Testament, of course.  So does it prove anything that the canonical books confirm themselves?

Another example of this circular reasoning in action is when the Reformed attempt to discredit, say Judith or Tobit, on account of its apparent use of literary convention rather than a strict record of actual history.  They say, ‘this is not found in the canon,’ but that begs the question.  The question at hand is whether or not the books are canonical!  How can you say it cannot be in the canon because nothing else like it is in the canon when we haven’t yet settled the question of the canon?  By that rationale, Hebrews is not canonical because nowhere else do we find Jesus called the High Priest.  And many more examples could be given.

To avoid this circularity in reason, as Tom Brown showed in his article on the canon, the canon must have an external standard for itself and this is incompatible with Protestant theology.