Half of Leibniz’ product was an argumentum ad hominem, undertaken more or less incidentally as a discussion of some other writer’s ideas. His greatest book, which grew to 590 pages, began in 1696 as a seven-page review of Locke’s Essay concerning Human Understanding (1690), known then to Leibniz only from an abstract in Le Clerc’s Bibliothèque universelle. When the Essay appeared in a French translation (1700), Leibniz reviewed it again for a German magazine. He was quick to recognize the importance of Locke’s analysis, and generously praised its style. In 1703 he set himself to comment on it chapter by chapter; it is these comments that constitute Leibniz’ Nouveaux Essais sur l’entendement humain (New Essays on Human Understanding). When he learned of Locke’s death (1704), he left the commentary unfinished. It was not published till 1765, too late to interfere with the pervasive influence of Locke upon Voltaire and other luminaries of the French Enlightenment, but in time to share in molding Kant’s epochalCritique of Pure Reason. It is among the most important productions in the history of psychology.
In form it is a dialogue between Philalethes (Lover of Truth), representing Locke, and Theophilus (Lover of God), representing Leibniz. The dialogue is vigorously sustained, and still makes good reading for any person of keen mind and endless leisure. The preface shows Leibniz in his most courteous mood, professing modestly to win readers by attaching his discourse to “the Essay on the Understanding by a distinguished Englishman, one of the most beautiful and esteemed works of this period.” The question to be discussed is stated with laudable clarity: “To know whether the soul in itself is entirely empty as the tablets upon which as yet nothing has been written (tabula rasa), according to Aristotle and the author of the Essay, and whether all that is traced thereon comes solely from the senses and experience; or whether the soul contains originally the principles of many ideas and doctrines which external objects merely call up on occasion, as I believe with Plato.” 33* The mind, in Leibniz’ view, is not a passive receptacle of experience; it is a complex organ that by its structure and functions transforms the data of sensation, just as the digestive tract is no empty sack but a system of organs for the digestion of food and its transformation into the needs and organs of the body. In a famous epigram Leibniz summarized and amended Locke: Nihil est in intellectu quod non fuerit in sensu, nisi ipse intellectus—“Nothing is in the mind that has not been in the senses, except the mind itself.” 36 Locke, as Leibniz noted, had recognized that ideas could come from introspective “reflection” as well as from external sensation, but had ascribed to a sensory origin all elements entering into reflection. Leibniz, on the contrary, argued that the mind of itself supplies certain principles or categories of thought, such as “being, substance, unity, identity, cause, perception, reason, and many other notions which the senses cannot give”; 37 and that these tools of understanding, these organs of mental digestion, are “innate,” not in the sense that we are conscious of them at birth, or always conscious of them when we use them, but in the sense that they are part of the native structure or “natural aptitudes” of the mind. Locke felt that these supposedly inborn principles were gradually developed by the interplay, in thought, of ideas originally sensory. But without such principles, Leibniz urged, there would be no ideas, only a disorderly succession of sensations; just as, without the action and digestive juices of the stomach, food would not feed us, and would not be food. In this measure, he boldly added, all ideasare innate—i.e., the result of the transforming action of the mind upon sensations. But he admitted that the innate principles are at birth confused and indistinct, and become clear only through experience and use.
The inherent principles, in Leibniz’ judgment, include all “necessary truths, such as are found in pure mathematics,” 38 for it is the mind, not sensation, that supplies the principle of necessity; everything sensory is individual and contingent, and gives us, at best, repeated sequence but not necessary sequence or cause. 39 (Locke had conceded this. 40) Leibniz considered innate all our instincts, our preference of pleasure to pain, and all the laws of reason 41—though these become clear only with experience. Among the innate laws of thought two are especially basic: the principle of contradiction—that contradictory statements cannot be true at the same time (“if A is a circle it is not a square”); and the principle of sufficient reason—“that nothing happens without a reason why it should be so rather than otherwise.” 42 Human intelligence, Leibniz thought, differs from animal knowledge by deducing general ideas from particular experiences through the use of innate principles of reason; brutes are pure empirics, guiding themselves solely by examples; “so far as we can judge of them, they never attain to the formation of necessary propositions.” 43
The principle of sufficient reason suffices to “demonstrate the being of a God, and all the other parts of metaphysics or natural theology.” 44 In this sense our idea of God is innate, though in some minds or tribes the idea may be unconscious or confused; and we may say the same of the idea of immortality. 45 The moral sense is innate, not in its specific content or judgments, which may vary in time and place, but as consciousness of a difference between right and wrong; this consciousness is universal. 46
In Leibniz’ psychology the mind is active not only as entering by its structure and functioning into the making of every idea, but also in the incessant continuance of its activity. Using the word think in Descartes’ broad sense as including all mental operations, Leibniz agreed with the Cartesians that the mind is always thinking, whether awake, unconscious, or asleep. “A state without thought in the soul, and an absolute repose in the body, appear to me equally contrary to nature, and without example in the world.” 47Some mental operations are subconscious; “it is a great error to believe that there is no perception in the soul besides those of which it is conscious.” 48 It is with such propositions in Leibniz that modern psychology began its efforts to delve into what some students called unconscious mind, and what esprits forts considered to be merely cerebral or other bodily processes that did not evoke consciousness.
Leibniz has much to say about the relation between body and soul, but there he leaves psychology, soars into metaphysics, and asks us to see all the world as psychophysical monads, like ourselves.