This text is generally held to be a touchstone of canonicity:
All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be complete, equipped for every good work.—2nd Timothy 3:16-17 (ESV)
I've always read this to mean that if a text is Scripture then it is inspired and useful for teaching, etc. In other words, the criteria is exclusive.
Tertullian, however, seems to see it as inclusive:
But since Enoch in the same Scripture has preached likewise concerning the Lord, nothing at all must be rejected by us which pertains to us; and we read that “every Scripture suitable for edification is divinely inspired.”
That is, if a text is suitable for edification then it is inspired and therefore Scripture. Of course, the canon was in flux at the time, so it wouldn't have been possible for the author of 2nd Timothy to unambiguously be referring to a specific set of texts here. But was Paul's intention be to open the door to any text that could be described as "profitable for teaching, for reproof, for correction, and for training in righteousness"?