The twenty-one philosophical personalities who represent an adventure in thinking are: Plato, Socrates, Aristotle, Epicurus, Marcus Aurelius, Thomas Aquinas, Francis Bacon, Descartes, Spinoza, Locke, Hume, Voltaire, Kant, Hegel, Schopenhauer, Emerson, Spencer, Nietzsche, William James, Henri-Louis Bergson, and Santayana.