哲學是愛智（熱愛智慧、追求知識、知天、知人、知己）的學問。康德認為在純粹理性指引下，遵循數理、邏輯與自然科學的方法，我們知識可以窮究經驗世界、是然世界（知悉已發生、正在發生、未來要發生）的自然、社會和人文現象。但現象背後的實體界(Noumena)也就是「物自身」卻不是理論哲學可以知悉的，這方面我們只能憑藉形而上學、神學、倫理學、政治學，一言以蔽之，實踐哲學，來加以瞭解和體驗。原因是作為擁有義務感與責任感的人類，有異於是然世界的其餘眾生，必須活在道德世界當中。道德是實踐的、是人的德行與理想的追求，也是對應然的、當為的行動的規範與約束。是故在自主、自由、自決中，人選擇了應為的行動。這點與認知和技術應用（狹義）的實踐不同。由是可知康德不但分辨（認知的）主體與客體、現象與本體、是然與應然，也分辨了理論與實踐。黑格爾則企圖把康德這種兩元對立的思考打破。對他而言，主體最終與客體聯合，是然與應然的鴻溝要填平，而理論與實踐也合一不但人在其心靈上要符合理性原則發展，就是外頭的世界最終也因人的認知、勞動、形塑而走上合理之途。馬克思更進一步把康德與黑格爾追求心靈與實在合理之理論，以批判和革命活動的實踐，使世界遭受改變（合理的改答）,從而人群與社會真正進入合理發展的階段。本文析述德國三位思想家怎樣從唯心主義轉向唯物主義的過程，特別指出理論與實踐怎樣經歷了分離、趨同和統一的過程。

This article deals with philosophical debate on theory and practice in the age of German Enlightenment. It focuses on the diverse and opposite viewpoints of Kant, Hegel and Marx. Kant's attempt to solve the problem "how are synthetic a priori judgment possible?", leads him to develop his theory of knowledge. According to him reality consists of appearances (phenomena) and things-in-themselves (noumena). What we know are only appearances, but things-in-themselves are unknowable. The latter can be he postulated and intuited, but not experienced by our senses. For Kant knowledge is a product of both sense and understanding which provides us with one kind of principles. This is due to the operation of pure reason, whose result is theoretical knowledge. On the other hand, there are also principles based on the concepts of reason which have their own kind of validity, and are not as theoretical knowledge but in terms of practice. This is the basis of Kant's bold separation of the world of theoretical knowledge and science and the practical world of God, morality and human action (especially, freedom of will and autonomy). Thus Kant distinguishes "sensible world" of scientific knowledge from the "intelligible world" of morality, freedom, and good intentions, God and the immortality of soul. Kant's schizoid two-standpoint, two-world, two-self view paves way for his dichotomy of to be and ought to be, necessity and freedom, theory and practice. Hegel tries to overcome Kant's dualism by stressing the desperate journey of consciousness, self-conscious, reason and spirit. Put in other words, he delineates a dialectical, ascending and transcending process of subjective through objective to absolute knowledge. In his major work Phenomenology of Spirit (1807), Hegel explicates the transition from consciousness to self-consciousness not a shift from realism to idealism, but from theory to practice, in theorizing we have a detached view of the world, and so abstract from our position as subjects in the world, whereas in practical activity we act on the world and put ourselves as subjects at the centre of things. Human theoretical attitude concerns their focus on the object, while in their practical attitude they subordinate object to the subject. In fact, Hegel sees no absolute gap between to be and ought to be, between necessity and freedom, between theory and practice. Marx further bridges the chasm between subject and object, necessity and freedom, theory and practice. For him practice means critical and revolutionary activity which determines human theoretical consciousness. On the other hand, there are some theories, especially empirical, techno-scientific and historical knowledge which constitute productive forces, thus guide our action. Such kind of theory is not limited and bound by practical activity. Finally, Marx's own materialist conception of history goes beyond the limitation of practice. For him the dialectical and historical materialism evidence the unity of theory and practice.