Critik der reinen Vernunft (Critique of Pure Reason).

"Kant's great achievement was to conclude finally the lines on which philosophical speculation had proceeded in the eighteenth century, and to open up a new and comprehensive system of dealing with the problems of philosophy ... The influence of Kant is paramount in the critical method of modern philosophy. No other thinker has been able to hold with such firmness the balance between speculative and empirical ideas. His penetrating analysis of the elements involved in synthesis, and the subjective process by which these elements are realized in the individual consciousness, demonstrated the operation of "pure reason"; and the simplicity and cogency of his arguments achieved immediate fame. Kant's achievements in other branches of philosophy were equally distinguished and fruitful... His methods... dominated western philosophical thought throughout the nineteenth century as they do today" (PMM).