Arthur Schopenhauer was a German philosopher that responded to and expanded upon Immanuel Kant's philosophy concerning the way in which we experience the world. His critique of Kant, his creative solutions to the problems of human experience and his explication of the limits of human knowledge are among his most important achievements. His metaphysical theory is the foundation of his influential writings on psychology, aesthetics, ethics, and politics which influenced Friedrich Nietzsche, Wagner, Ludwig Wittgenstein, Sigmund Freud and others. His philosophy was influenced by the Upanishads, Immanuel Kant, and Plato. He appreciated the teachings of the Buddha and even called himself a Buddhaist, and had References to Eastern philosophy and religion appearing frequently in his writings. He said that his philosophy could not have been conceived before these teachings were available.