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 PP‘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.

According to Leibniz, what appear to be real causal relations between mind and body are, in metaphysical reality, the mutual conformity or coordination of mind and body—in accordance with (3)—with no interaction or divine intervention involved. For example, suppose that Smith is pricked with a pin (call this bodily state Sb) and pain ensues (call this mental state Sm), a case of apparent body to mind causation. Leibniz would say that in such a case some state of Smith’s mind (soul) prior to Sm was the real cause of Sm, and Sb was not a causal factor in the obtaining of Sm. Suppose now that Smith has a desire to raise his arm (call this mental state Sm), and the raising of his arm ensues (call this bodily state Sb), a case of apparent mind to body causation. Leibniz would say that in such a case some state of Smith’s body prior to Sb was the real cause of Sb and Sm was not a causal factor in the obtaining of Sb. So although substances do not causally interact, their states accommodate one another as if there were causal interaction among substances.

It should be noted, however, that Leibniz did think that there was a sense in which one could say that mental events influence bodily events, and vice-versa. He wrote to Antoine Arnauld that although “one particular substance has no physical influence on another … nevertheless, one is quite right to say that my will is the cause of this movement of my arm …; for the one expresses distinctly what the other expresses more confusedly, and one must ascribe the action to the substance whose expression is more distinct” (28 November 1686 (draft)). In this passage, Leibniz sets forth what he takes the metaphysical reality of apparent inter-substantial causation to amount to. We begin with the thesis that every created substance perceives the entire universe, though only a portion of it is perceived distinctly, most of it being perceived unconsciously, and, hence, confusedly. Now consider two created substances, x and y (x not identical to y), where some state of x is said to be the cause of some state of y. Leibniz’s analysis is this: when the causal state of affairs occurred, the relevant perceptions of substance x became more distinct, while the relevant perceptions of substance y became more confused. Insofar as the relevant perceptions of x become increasingly distinct, it is “causally” active; insofar as the relevant perceptions of substance y become increasingly confused, it is passive. In general, causation is to be understood as an increase in distinctness on the part of the causally active substance, and an increase in confusedness on the part of the passively effected substance. Again, each substance is programmed at creation to be active/passive at the relevant moment, with no occurrence of real substantial interaction.

It is difficult to say exactly why Leibniz denied inter-substantial causation. Some of the things he tells us, in both private and public writings, seem unsatisfactory for one reason or another. For example, in Primary Truths (1686?), we are given this:

Strictly speaking, one can say that no created substance exerts a metaphysical action or influx on any other thing. For, not to mention the fact that one cannot explain how something can pass from one thing into the substance of another, we have already shown that from the notion of each and every thing follows all of its future states. What we call causes are only concurrent requisites, in metaphysical rigor.Here Leibniz gives a reason tied to his complete concept theory of substance, according to which “the nature of an individual substance or of a complete being is to have a notion so complete that it is sufficient to contain and to allow us to deduce from it all the predicates of the subject to which this notion is attributed” (Discourse on Metaphysics, sec. 8). But there are, it seems, at least two problems with this explanation. First, Leibniz moves rather quickly from a conceptual explanation of substance in terms of the complete concept theory, to the conclusion that this consideration is sufficient to explain the activity of concrete substances. Second, even if conceptual considerations about substances were sufficient to explain their apparent causal activity, it does not seem to follow that substances do not interact—unless one is assuming that causal overdetermination is not a genuine possibility. Leibniz seems to be assuming just that, but without argument.

Sometimes Leibniz gives a more familiar line of reasoning. At Monadology 7, we read this:

There is no way of explaining how a monad can be altered or changed internally by some other creature, since one cannot transpose anything in it, nor can one conceive of any internal motion that can be excited, directed, augmented, or diminished within it, as can be done in composites, where there can be change among the parts. The monads have no windows through which something can enter or leave. Accidents cannot be detached, nor can they go about outside of substances, as the sensible species of the Scholastics once did. Thus, neither substance nor accident can enter a monad from without.He seems to think that causal interaction between two beings requires the transmission or transposition of the parts of those beings. But substances are simple unextended entities which contain no parts. Thus, there is no way to explain how one substance could influence another. Unfortunately, however, this line of reasoning would seem to also rule out one case of inter-substantial causation which Leibniz allows, viz., God’s causal action on finite simple substances.

Occaisonalism (http://www.britannica.com/EBchecked/topic/424157/occasionalism)

occasionalism, version of Cartesian metaphysics that flourished in the last half of the 17th century, in which all interaction between mind and body is mediated by God. It is posited that unextended mind and extended body do not interact directly. The appearance of direct interaction is maintained by God, who moves the body on the occasion of the mind’s willing and who puts ideas in the mind on the occasion of the body’s encountering other material objects. For example, when a person actualizes his desire to pick up an apple, his mind does not act on his body directly, but his willing of the action is the occasion for God to make his arm reach out; and when his hand grasps the apple, the apple does not act on his mind directly, but the contact is the occasion for God to give him ideas of the apple’s coolness and softness.

eveloped primarily by Arnold Geulincx and Nicolas Malebranche, 17th-century Dutch and 17th–18th-century French philosophers, respectively, to solve a specific problem in Cartesian metaphysics. For René Descartes, mind is active, unextended thinking, whereas body is passive, unthinking extension. But these two created substances, the bases of Cartesian dualism, are combined as a third, compound substance—the living human. The problem is that the essential unlikeness of mind and body in the Cartesian view makes it difficult to conceive how they can interact—i.e., how unextended mental ideas can push the body around and how bodily bumpings can yield ideas. Descartes’s opinion that direct interaction takes place in the pineal gland deep within the brain does not answer the question of how. The orthodox view of the French Cartesians Pierre-Sylvain Régis and Jacques Rohault was simply that God has made mind and body so that they interact directly even if scientists do not know how. The occasionalist’s answer to the question is to show how interaction appears to be direct when in fact it is mediated by the fourth, uncreated Cartesian substance, God.


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