Immanuel Kant
German philosopher Immanuel Kant's position as one of the greats in Western metaphysics comes from works he published late in life, including Critique of Pure Reason (1781) and The Fundamental Principles of the Metaphysics of Ethics (1785). He grew up in Königsberg and never really left home, spending most of his professional life attached to the university there as a lecturer on philosophy. His lectures made him internationally famous, and he followed an outstanding academic career with published works that became standards in Western philosophy. Kant, influenced by the works of David Hume, held that we could only know what we experience, what he called the phenomenal, and that we could never know that which is beyond experience, what he called the noumenal. In doing so, Kant ruled out the possibility of our demonstrable knowledge of God, without ruling out the existence of God. But he also argued for an absolute morality based on free will and rationality, referred to as the categorical imperative: "Act as if the maxim from which you act were to become through your will a universal law." His brilliant, sometimes impenetrable arguments about the limits of human understanding are part of the canon of Western thought. His works include Prolegomena (his 1783 attempt to better explain Critique of Pure Reason), Critique of Practical Reason (1788) and Critique of Judgment (1790).
Extra credit: Kant's belief that our understanding of objects would always be skewed by our limited capacity to understand can be seen in his famous quote: "Out of the crooked timber of humanity no straight thing can ever be made."