His second response was more considered and very courageous. He confronted his long-held convictions and examined them on possible flaws, by which meant to determine which of the claims previously advanced hold up and those that do not. This soul-searching amounted to a second-order investigation—not an examination of the conceptual contentions as such, but an examination of their knowability. This turned out to be an intense task that consumed more than a decade of his life, and which resulted in his greatest work, the Critique of Pure Reason (1781).