The German philosopher, mathematician, and statesman Gottfried Wilhelm Leibniz developed a remarkably subtle and original system of philosophy. It combined the mathematical and physical discoveries of his time with the organic and religious conceptions of nature found in ancient and medieval thought. Leibniz viewed the world as an infinite number of infinitely small units of force, called monads, each of which is a closed world but mirrors all the other monads in its own system of perceptions. All the monads are spiritual entities, but those with the most confused perceptions form inanimate objects and those with the clearest perceptions, including self-consciousness and reason, constitute the souls and minds of humanity. God is conceived of as the Monad of Monads, who creates all other monads and predestines their development in accordance with a preestablished harmony that results in the appearance of interaction between the monads. Leibniz's view that all things are organic and spiritual initiated the philosophical tradition of idealism.
In answer to the skepticism of Hume, the German philosopher Immanuel Kant constructed a comprehensive system of philosophy that ranks among the greatest intellectual achievements in Western culture. Kant combined the empiricist principle that all knowledge has its source in experience with the rationalist belief in knowledge obtained by deduction. He suggested that although the content of experience must be discovered through experience itself, the mind imposes form and order on all its experiences, and this form and order can be discovered a priori, that is, by reflection alone. His claim that causality, substance, space, and time are forms imposed by the mind on its experience gave support to the idealism of Leibniz and Berkeley, but he made his view a more critical form of idealism by granting the empiricist claim that things-in-themselves, that is, things as they exist outside human experience, are unknowable. Kant therefore limited knowledge to the “phenomenal world” of experience, maintaining that metaphysical beliefs about the soul, the cosmos, and God (the “noumenal world” transcending human experience) are matters of faith rather than of scientific knowledge. In his ethical writings Kant held that moral principles are categorical imperatives, absolute commands of reason that permit no exceptions and are not related to pleasure or practical benefit. In his religious views, which had a lasting effect on Protestant theology, he emphasized individual conscience and represented God primarily as a moral ideal. In political and social thought Kant was a leading figure of the movement for reason and liberty against tradition and authority.
In France, intellectual activity culminated in the period known as the Enlightenment, which helped stimulate the social changes that produced the French Revolution. Among the leading thinkers of this period were Voltaire, who, developing the tradition of DEISM, (q.v.) begun by Locke and other liberal thinkers, reduced religious beliefs to those that can be justified by rational inference from the study of nature; Jean Jacques Rousseau, who criticized civilization as a corruption of humanity's nature and developed Hobbes's doctrine that the state is based on a social contract with its citizens and represents the popular will; and Denis Diderot, who founded the famous Encyclopédie, to which many scientists and philosophers contributed.
In Germany, through the influence of Kant, idealism and voluntarism, that is, emphasis on the will, became the dominant tendencies. Johann Gottlieb Fichte transformed Kant's critical idealism into absolute idealism by eliminating Kant's “things-in-themselves” and making the will the ultimate reality. Fichte maintained that the world is created by an absolute ego, of which the human will is a partial manifestation and which tends toward God as an unrealized ideal. Friedrich Wilhelm Joseph von Schelling went still further in reducing all things to the self-realizing activity of an absolute spirit, which he identified with the creative impulse in nature. The emphasis of romanticism on feeling and on the divinity of nature found philosophical expression in the thought of Schelling, who influenced the American transcendentalist movement, led by the poet and essayist Ralph Waldo Emerson.
The most powerful philosophical mind of the 19th century was the German philosopher Georg Wilhelm Friedrich Hegel, whose system of absolute idealism, although influenced greatly by Kant and Schelling, was based on a new conception of logic in which conflict and contradiction are regarded as necessary elements of truth, and truth is regarded as a process rather than a fixed state of things. The source of all reality, for Hegel, is an absolute spirit, or cosmic reason, which develops from abstract, undifferentiated being into more and more concrete reality by a dialectical process consisting of triadic stages, each triad involving an initial state (or thesis), its opposite state (or antithesis), and a higher state, or synthesis, that unites the two opposites. According to this view, history is governed by logical laws, so that “all that's real is rational, and all that's rational is real.” Later historical forms are more concrete fulfillments of the absolute spirit, whose highest stage of self-realization is found in the national state and in philosophy. Hegel stimulated greater interest in history by representing it as a deeper penetration into reality than natural science. His conception of the national state as the highest social embodiment of the absolute spirit was for some time believed to be a main source of modern totalitarian ideologies, although Hegel himself argued for a large measure of individual freedom.
Other influential philosophers.
The German philosopher Arthur Schopenhauer rejected the optimistic faith of Hegel in reason and progress. Schopenhauer maintained that both nature and humanity are products of an irrational will, from which people can escape only through art and through philosophical renunciation of the desire for happiness. The French mathematician and philosopher Auguste Comte formulated the philosophy of positivism, which rejected metaphysical speculation and located all genuine knowledge in the so-called positive, or factual, sciences. Comte placed the science of sociology, which he founded, at the top of his classification of the sciences. The British economist John Stuart Mill developed and refined the empiricist and utilitarian traditions, applying their principles to all fields of thought. Mill and other utilitarians influenced liberal social and economic reforms in Great Britain. The Danish mystic Søren Kierkegaard attacked the Hegelian emphasis on reason, and his eloquent defense of feeling and of a subjective approach to the problems of life became one of the main sources of 20th-century existential philosophy.
The mechanistic world view of the 17th century and the faith in reason and common sense of the 18th century, although still influential, were modified in the 19th century by a variety of more complex and dynamic views, based more on biology and history than on mathematics and physics. Particularly influential was the theory of evolution through natural selection, proposed by Charles Darwin, whose work inspired conceptions of nature and of humanity that emphasized conflict and change, as against unity and substantial permanence. The German revolutionists Karl Marx and Friedrich Engels developed the philosophy of dialectical materialism, based on the dialectical logic of Hegel, but they made matter, rather than mind, the ultimate reality. They derived from Hegel the belief that history unfolds according to dialectical laws and that social institutions are more concretely real than physical nature or individual mind. Their application of these principles to social problems took the form of historical materialism, the theory that all forms of culture are determined by economic relations and that social evolution proceeds through class conflict and periodic revolutions. This theory became the ideological basis for the Communist movement. The British philosopher Herbert Spencer developed an evolutionary philosophy based on the principle of “the survival of the fittest,” which explains all elements of nature and society as adaptations in the cosmic struggle for survival. Like Comte, he based philosophy on sociology and history, as the highest sciences.
The German philosopher Friedrich Wilhelm Nietzsche returned to Schopenhauer's conception of life as the expression of a cosmic will, but he made the so-called will to power the source of all value. He called for a return from religious ethics to the more primitive and natural virtues of courage and strength. Continuing the romantic revolt against reason and social organization, he stressed the values of individual self-assertion, biological instinct, and passion.