To the philosopher the main interest of Barth lies in his apparent irrationalism. He has had the courage to break with philosophy frankly and thoroughly, and to insist that the knowledge of God, though the most important of all knowledge, is not in the ordinary sense knowledge at all, nor, therefore, subject to the sort of criticism or support that reason has to offer. This is, of course, not a new position in either philosophy or theology. Mystics have often held that they had a clear and certain knowledge of God that was above reason and incapable of expression in thought or speech. In Christian theology the claim of a non-rational knowledge of God is as old as Tertullian; it was accepted by Pascal in his doctrine of "reasons of the heart that the reason knows not of"; it was developed with vehement verbosity by Kierkegaard, and with logic and eloquence by Dean Mansel. Though Barth lacks the acute-ness, scholarship and style of Mansel, there is a Lutheran self-confidence, energy and pugnacity about him that has caught the religious world's attention, and given him the best-known name among living theologians.