As introductory to the following dissertation, I shall explain and define certain terms that frequently occur in it, especially _canon_, _apocryphal_, _ecclesiastical_, and the like. A right apprehension of these will make the observations advanced respecting the canon and its formation plainer. The words have not been taken in the same sense by all, a fact that obscures their sense. They have been employed more or less vaguely by different writers. Varying ideas have been attached to them. The Greek original of _canon_(1) means primarily a straight rod or pole; and metaphorically, what serves to keep a thing upright or straight, a _rule_. In the New Testament it occurs in Gal. vi. 16 and 2 Cor. x. 13, 15, 16, signifying in the former, a measure; in the latter, what is measured, a _district_. But we have now to do with its ecclesiastical use. There are three opinions as to the origin of its application to the writings used by the church. According to Toland, Whiston, Semler, Baur, and others, the word had originally the sense of _list_ or _catalogue_ of books publicly read in Christian assemblies. Others, as Steiner, suppose that since the Alexandrian grammarians applied it to collections of Old Greek authors as _models_ of excellence or _classics_, it meant _classical_ (canonical) _writings_. According to a third opinion, the term included from the first the idea of a regulating principle. This is the more probable, because the same idea lies in the New Testament use of the noun, and pervades its applications in the language of the early Fathers down to the time of Constantine, as Credner has shown.(2) The “canon of the church” in the Clementine homilies;(3) the “ecclesiastical canon,”(4) and “the canon of the truth,” in Clement and Irenæus;(5) the “canon” of the faith in Polycrates,(6) the _regula fidei_ of Tertullian,(7) and the _libri regulares_ of Origen,(8) imply a _normative principle_. But we cannot assent to Credner’s view of the Greek word for _canon_ being an abbreviation of “Scriptures of canon,”(9) equivalent to _Scripturæ legis_ in Diocletian’s Act(10)—a view too artificial, and unsanctioned by usage. It is true that the word _canon_ was employed by Greek writers in the sense of a mere _list_; but when it was transferred to the Scripture books, it included the idea of a regulative and normal power—a list of books forming a rule or law, because the newly-formed Catholic Church required a standard of appeal in opposition to the Gnostics with their arbitrary use of sacred writings. There is a lack of evidence on behalf of its use before the books of the New Testament had been paralleled with those of the Old in authority and inspiration.