Protecting reasoning against fallacy is not a demand to doubt everything. It is a way to distinguish genuine grounds for knowledge from the appearance of certainty. Even a claim that feels obvious may require inspection of its terms, scope and warrant.
Before labeling a disputed claim a first principle, common opinion, accepted report or concession, we must verify the relevant condition. The necessary connection from the premises to the conclusion must then be checked separately.
This completes the purpose of logic introduced at the beginning of the book: guarding thought from avoidable error.