Arguably Kant's most important achievements have been (i) his critical method, (ii) his answer to the questions: What can I know?, (iii) What ought I to do?, (iv) What can I hope for?, (v) How can I find undisturbed happiness? These are also the problems on which the author has focused. In the first place he emphasises the distinction introduced by Kant between thought and perception, which in itself was a consequence of Kant's belief that our knowledge arises from two different sources, and thought without perception is empty, while perception without mental contents is blind. This is an occasion for the author to highlight the active role of the intellect in cognition and to show that human knowledge is limited to the cognition of phenomena rather than things in themselves. The author explains subsequently what is the origin and role of the categories, concepts and ideas in Kant's system of transcendental idealism. Along with the problems of theoretical philosophy he also deals with some issues of practical philosophy (autonomy, rationality, the formal ethics of obligation), and religion. He also shows why when carrying out the project of transcendental idealism Kant was obliged to recognise the a priori character of fundamental rules and laws. Finally the question of Kant's moral postulates is discussed. Man should treat another man always as an end in itself, and never only as a means, political society should be based on just laws and should be governed with fairness by its rulers, happiness is to be found in what is sublime and beautiful. The author points out in this context how Kant's philosophy has influenced Catholic philosophical thought of the 20th century (Mueller, Krings, Baumgartner).