[Dr. Eckleburg]

Surely this important principle is itself written down somewhere?

Surely.

"All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:

That the man of God may be perfect, throughly furnished unto all good works." -- 2 Timothy 3:16-17

[annalex]

2 Timothy 3:16-17 says that "scripture alone contains all that God has revealed for the salvation of man and fellowship with Him"? Where?