Philosophers use various methods to express their philosophy. The dialogue is the oldest method, used by the ancient Greek thinkers like Socrates and Plato. Aristotle does not use the dialogue method—his philosophy comes in the form of lecture notes. Several works of Hegel and Heidegger are in the form of lecture notes. Parmenides and Lucretius use the poetic method. Descartes and Spinoza use the mathematical method. Augustine uses the autobiographical method. In the modern age, some philosophers have devoted years, or even decades, of their life to develop a system of philosophy. Kant’s three Critiques and his works on ethics constitute a philosophical system. Hegel has produced systematic philosophy through multiple works. Schopenhauer devoted much of his life to producing a single work of systematic philosophy, The World as Will and Representation. Sartre’s Being and Nothingness too is a work of systematic philosophy. Cicero, Aquinas, Bacon, Machiavelli, Leibniz, and Rousseau have produced long essays and books, but their work is not systematized—the same is the case with the works of philosophers like MacIntyre and Strauss. Seneca, Aurelius, Voltaire, Kierkegaard, and Nietzsche. Russell, Wittgenstein, Camus, Derrida, and Foucault have philosophized through long and short essays. Several incomplete philosophical works have become immensely influential: for example, Plato’s Critias, Pascal’s Pensées, Marx’s Capital, Heidegger’s Being and Time.