At the end of a life spent in the pursuit of knowledge Faust has to confess: "I now see that we can nothing know." That is the answer to a sum, it is the outcome of a long experience. But as Kierkegaard observed, it is quite a different thing when a freshman comes up to the university and uses the same sentiment to justify his indolence. As the answer to a sum it is perfectly true, but as the initial data it is a piece of self-deception.
translated as The Cost of Discipleship (1959), p. 51. - Discipleship (1937) - Costly Grace