PRE-ESTABLISHED HARMONY. — This is a doctrine—or hypothesis—central to Leibniz, which, although it has various historical roots (Pythagorean harmony, Platonic mimesis of ideas in things, the scholastic doctrine of analogy, the problem of predestination in seventeenth-century theology, etc.), is specifically grounded in the framework of Cartesian philosophy. Descartes, in fact, with his dualism of res cogitans and res extensa, had split the human substantial unity into two complete substances, soul and body, each with its own perfectly heterogeneous attributes and laws. Their reciprocal influence thus became impossible, and there arose the problem of rationally explaining that relationship, which experience nevertheless attests to us. Working on this Cartesian basis, Malebranche had appealed to the causality of God, operating in bodies and spirits ad occasione of certain modifications of theirs (occasionalism, v.); Spinoza, meanwhile, had concluded in the pantheistic monism of a single substance, expressing itself and being conceivable under different attributes, whose modifications correspond to one another in parallel series (parallelism of the modes of extension and thought). Leibnizian speculation introduces critical corrections and contributions of original motifs. It criticizes Descartes, radicalizing his demands through the observation that the soul can not only fail to increase the quantity of motion existing in bodies, but cannot even cause its direction to vary; it supplements him, while surpassing his mechanism and rationalism, by asserting the unity and purposiveness of the organic body and by expanding the intellectualist mens into a soul also endowed with subconsciousness. It criticizes Malebranche, observing to him that this recourse, case by case, to the direct intervention of God postulates more or less a continual miracle. Against Spinoza it asserts the undeniable existence of a pluralism of active or conscious centres, that is, of substantial and microcosmic monads. The conviction nevertheless remains unshaken—metaphysical and methodological—that influence between monad and monad, or between mechanical laws and vital and spiritual laws, is impossible. Thus, in order to explain the consensus or correspondence between body and soul and between monad and monad, there remains only the hypothesis of a pre-established harmony: God, that is, so arranged from the beginning, once and for all, the elements and the nexus of the existing world that every monad, at every moment of its independent and spontaneous development, corresponds with the contemporary state of the universe (one should recall the comparison with the clocks, somewhat mechanical, but precisely for that reason significant).
It is easy to imagine what discussions this doctrine must have provoked: discussions that continued in various directions for more than three decades in the world of philosophers and theologians. The principal difficulty raised against it arose from the concern to safeguard the freedom of the human will, effectively compromised by this perspective, governed by a rigorous yet refined rationalistic mentality. Soon, however, after the middle of the eighteenth century, the accusations, defences, and attempts at contamination and reconciliation by the epigones were supplanted by a transfiguration of the problem, brought about by the transformation of Leibnizian rationalism into idealism. In Kant and in the great systematic thinkers of idealism (Fichte, Schelling, Hegel), with the subjectivization of the corporeal world and the absorption of the monadic selves into the transcendental self, the very terms of pre-established harmony were emptied and disappeared; nevertheless, the problems of pre-established harmony reappeared, enriching idealistic transcendental subjectivity and almost overcoming certain of its dead ends (especially in Kant’s Critique of Judgment and in Schelling).