Immanuel Kant, an influential philosopher of the 18th century, played a crucial role in the development of modern philosophical thought. In his most famous work, Critique of Pure Reason, Kant explores the limits and possibilities of human knowledge and introduces the idea of transcendental philosophy. A core principle of his philosophy is the notion of the "thing-in-itself," which states that human experience can only know the world through the structures and conditions of human perception. Thus, the "thing-in-itself" - that which exists independently of our perception - remains inaccessible to the human mind. In addition, Kant also shaped moral philosophy through his Categorical Imperative, which serves as the basis for ethical action. This principle states that actions are moral only if they are based on a maxim that can be universalized. This means that individual actions are considered ethical only if they can serve as a guiding principle for all people equally.