Adam Clarke on 2 Peter 1:20

Knowing this first - Considering this as a first principle, that no prophecy of the Scripture, whether that referred to above, or any other, is of any private interpretation - proceeds from the prophet's own knowledge or invention, or was the offspring of calculation or conjecture. The word επιλυσις signifies also impetus, impulse; and probably this is the best sense here; not by the mere private impulse of his own mind.