Immanuel Kant (1724–1804) responded to the standoff between rationalism and empiricism with what he called critical philosophy. In the Critique of Pure Reason, he argued that although knowledge begins with experience, its structure is shaped by the mind's a priori categories. He drew a sharp line between phenomena, things as they appear to us through sensory experience, and noumena, things-in-themselves, which we can never directly know. Central to his epistemology are synthetic a priori judgments, which are both informative and knowable independent of experience — mathematical truths like "7 + 5 = 12" being paradigm cases. For Kant, this category rescues both empirical science and metaphysics from the threats of pure rationalism and pure skepticism.
Kant's ethics, presented in works like the Groundwork of the Metaphysics of Morals, rests on the Categorical Imperative: "Act only according to that maxim whereby you can at the same time will that it should become a universal law." Moral actions, on his view, must be done from duty alone (Pflicht), not from inclination or self-interest; only actions motivated by respect for the moral law carry genuine moral worth. The related ideal of the Kingdom of Ends envisions a moral community in which all rational beings treat each other as ends in themselves, never merely as means.
Georg Wilhelm Friedrich Hegel (1770–1831) carried Kantian idealism further with his dialectical method, in which contradiction drives progress. Every position (thesis) provokes its opposite (antithesis), and the two together yield a higher unity (synthesis). Hegel named the moving force behind history Geist, a collective rational Spirit that reaches self-awareness through human culture, art, religion, and philosophy. In the Phenomenology of Spirit, consciousness develops from immediate sense-certainty to absolute knowledge through a series of dialectical stages, including the famous master-slave dialectic, in which self-consciousness emerges through a struggle for recognition between two beings. Hegel's philosophy of history treats the entire sweep of human events as the rational progress of Spirit toward freedom, unfolding through dialectical conflict among nations and ideas.