There are several synonyms for the term "enigmatical canon." The phrase refers to a rule or guideline that is difficult to interpret or understand. Some possible synonyms include puzzle rule, cryptic principle, perplexing guideline, or mysterious regulation. These terms all suggest an element of puzzlement or confusion, and indicate that the guideline or rule in question is not easily understood or deciphered. Other possible synonyms might include abstruse precept, baffling directive, or incomprehensible axiom. Whatever term is used, the underlying idea is the same -- that the rule or guideline in question requires careful consideration and interpretation in order to be fully understood.