In a more popular view, Leibniz's place in the history of the philosophy of mind is best secured by his pre-established harmony, that is, roughly, by the thesis that there is no mind-body interaction strictly speaking, but only a non-causal relationship of harmony, parallelism, or correspondence between mind and body. Certainly, the pre-established harmony is important for a proper understanding of Leibniz's philosophy of mind, but there is much more to be considered as well, and even in connection with the pre-established harmony, the more popular view needs to be refined, particularly insofar as it suggests that Leibniz accepts a roughly Cartesian, albeit non-interactionist dualism, which he does not. In fact, Leibniz is justly famous for his critiques, not only of materialism, but also of such a dualism. (Whether Leibniz accepts, throughout his maturity, the idealistic view that all substances are simple unextended substances or monads is an important interpretive issue that has been discussed widely in recent years. We shall not try to resolve the issue here. (See Garber 2009 for comprehensive treatment.) In short, Leibniz made important contributions to a number of classical topics of the philosophy of mind, including materialism, dualism, idealism and mind-body interaction.
But Leibniz has much to say about the philosophy of mind that goes well beyond these traditionally important topics. Perhaps surprisingly, his system sometimes contains ideas of relevance even to contemporary discussions in the cognitive sciences. More generally, he discusses in depth the nature of perception and thought (conscious and unconscious), and of human motivation and striving (or, as he would say, appetition).
1. Matter and Thought
For present purposes, we may think of materialism as the view that everything that exists is material, or physical, with this view closely allied to another, namely, that mental states and processes are either identical to, or realized by, physical states and processes. Leibniz remained opposed to materialism throughout his career, particularly as it figured in the writings of Epicurus and Hobbes. The realms of the mental and the physical, for Leibniz, form two distinct realms—but not in a way conducive to dualism, or the view that there exists both thinking substance, and extended substance. By opposing both materialism and dualism, Leibniz carved himself an interesting place in the history of views concerning the relationship between thought and matter.
Most of Leibniz's arguments against materialism are directly aimed at the thesis that perception and consciousness can be given mechanical (i.e. physical) explanations. His position is that perception and consciousness cannot possibly be explained mechanically, and, hence, could not be physical processes. His most famous argument against the possibility of materialism is found in section 17 of the Monadology (1714):
2. Denial of Mind-Body Interaction, Assertion of Pre-established Harmony
A central philosophical issue of the seventeenth century concerned the apparent causal relations which hold between the mind and the body. In most seventeenth-century settings this issue was discussed within the context of substance dualism, the view that mind and body are different kinds of substance. For Leibniz, this is a particularly interesting issue in that he remained fundamentally opposed to dualism. But although Leibniz held that there is only one type of substance in the world, and thus that mind and body are ultimately composed of the same kind of substance (a version of monism), he also held that mind and body are metaphysically distinct. There are a variety of interpretations of what this metaphysical distinctness consists in for Leibniz, but on any plausible interpretation it is safe to assume (as Leibniz seems to have done) that for any person P, P‘s mind is a distinct substance (a soul) from P‘s body. With this assumption in hand, we may formulate the central issue in the form of a question: how is it that certain mental states and events are coordinated with certain bodily states and events, and vice-versa? There were various attempts to answer this question in Leibniz's time period. For Descartes, the answer was mind-body interactionism: the mind can causally influence the body, and (most commentators have held) vice-versa. For Malebranche, the answer was that neither created minds nor bodies can enter into causal relations because God is the only causally efficient being in the universe. God causes certain bodily states and events on the occasion of certain mental states and events, and vice-versa. Leibniz found Descartes' answer unintelligible (cf. Theodicy, sec. 60), and Malebranche's excessive because miraculous (cf. Letter to Arnauld, 14 July 1686).
Leibniz's account of mind-body causation was in terms of his famous doctrine of the preestablished harmony. According to the latter, (1) no state of a created substance has as a real cause some state of another created substance (i.e. a denial of inter-substantial causality); (2) every non-initial, non-miraculous, state of a created substance has as a real cause some previous state of that very substance (i.e. an affirmation of intra-substantial causality); and (3) each created substance is programmed at creation such that all its natural states and actions are carried out in conformity with all the natural states and actions of every other created substance.
Formulating (1) through (3) in the language of minds and bodies, Leibniz held that no mental state has as a real cause some state of another created mind or body, and no bodily state has as a real cause some state of another created mind or body. Further, every non-initial, non-miraculous, mental state of a substance has as a real cause some previous state of that mind, and every non-initial, non-miraculous, bodily state has as a real cause some previous state of that body. Finally, created minds and bodies are programmed at creation such that all their natural states and actions are carried out in mutual coordination.
3. Language and Mind
Some scholars have suggested that Leibniz should be regarded as one of the first thinkers to envision something like the idea of artificial intelligence (cf. Churchland 1984; Pratt 1987). Whether or not he should be regarded as such, it is clear that Leibniz, like contemporary cognitive scientists, saw an intimate connection between the form and content of language, and the operations of the mind. Indeed, according to his own testimony in the New Essays, he “really believe
that languages are the best mirror of the human mind, and that a precise analysis of the signification of words would tell us more than anything else about the operations of the understanding” (bk.III, ch.7, sec.6 (RB, 333)). This view of Leibniz's led him to formulate a plan for a “universal language,” an artificial language composed of symbols, which would stand for concepts or ideas, and logical rules for their valid manipulation. He believed that such a language would perfectly mirror the processes of intelligible human reasoning. It is this plan that has led some to believe that Leibniz came close to anticipating artificial intelligence. At any rate, Leibniz's writings about this project (which, it should be noted, he never got the chance to actualize) reveal significant insights into his understanding of the nature of human reasoning. This understanding, it turns out, is not that different from contemporary conceptions of the mind, as many of his discussions bear considerable relevance to discussions in the cognitive sciences.
4. Perception and Appetition
What do we find in the human mind? Representations on the one hand, and tendencies, inclinations, or strivings on the other, according to Leibniz. Or, to put this in Leibniz's more customary terminology, what is found within us is perception and appetition. For human minds count for Leibniz as simple substances, and, as he says in a letter to De Volder, “it may be said that there is nothing in the world except simple substances, and, in them, perception and appetite.” (30 June 1704)
Perception has already been discussed briefly above. But it will be advisable to consider also a definition from a letter to Des Bosses (and echoed in many other passages), in which Leibniz discusses perception as the representation or “expression” of “the many in the one” (letter to Des Bosses, 11 July 1706). We shall return to this definition below. Appetitions are explained as “tendencies from one perception to another” (Principles of Nature and Grace, sec.2 (1714)). Thus, we represent the world in our perceptions, and these representations are linked with an internal principle of activity and change (Monadology, sec.15 (1714)) which, in its expression in appetitions, urges us ever onward in the constantly changing flow of mental life. More technically explained, the principle of action, that is, the primitive force which is our essence, expresses itself in momentary derivative forces involving two aspects: on the one hand, there is a representative aspect (perception), by which that the many without are expressed within the one, the simple substance; on the other, there is a dynamical aspect, a tendency or striving towards new perceptions, which inclines us to change our representative state, to move towards new perceptions. (See Carlin 2004.)