Immanuel Kant (1724-1804) was a German philosopher of the Enlightenment best known for his work on ethics, epistemology, and metaphysics. His three publications, Critique of Pure Reason, Critique of Practical Reason, and Critique of the Power of Judgement, form the basis for his theory of human rationalism and empiricism. Kant argues that knowledge is fundamentally a result of human reasoning and human experience. His philosophy is influenced by other Enlightenment figures, such as René Descartes and Isaac Newton.

Kant’s work has subsequently influenced other great modern thinkers, including Karl Marx, Rudolf Carnap, and Karl Popper himself. He figures in Conjectures and Refutations both as an example of the uses of empiricism and as the basis for the defense of pure reasoning. Popper believes that both empirical observation and pure reasoning play a role in the generation of human knowledge.

Isaac Newton (1643-1727) was an English astronomer and mathematician best known for having theorized about the three laws of motion, which remain to this day the foundational principles of modern physics. His work improves upon the scientific discoveries of Galileo Galilei, Johannes Kepler, and Nicolaus Copernicus by proving the solar system’s heliocentricity and by providing a means to calculate the laws of planetary motion.