We have seen that there are two principles of certainty, one intrinsic, the other extrinsic. The intrinsic principle is the intuitive knowledge of truth; the extrinsic, the knowledge of a sure sign of truth.
The extrinsic principle is never the ultimate principle; it is ordered to, and dependent on the intrinsic principle because a sure sign of truth cannot exist without a preceding certainty. And this certainty can only be given in the last analysis by the intuitive knowledge of truth....
Hence, the supreme or ultimate principle of certainty is one only, the intuition or vision of truth.
Antonio Rosmini, Certainty (New Essay on the Origin of Ideas, Volume III), Part I, Chapter 5.