Kant's Philosophy of Mind

 

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[FIRST DRAFT – COMMENTS WELCOME

NOTE: THIS DRAFT CONTAINS PART I, ON KANT’S PRE-CRITICAL PHILOSOPHY OF MIND. PARTS II (ON KANT’S CRITICAL PHILOSOPHY OF MIND) AND III (AN ANNOTATED BIBLIOGRAPHY) ARE IN PROGRESS.]

PART I: KANT'S PRE-CRITICAL PHILOSOPHY OF MIND

Although his critical views are more well-known, this tour will provide a detailed survey of Kant’s pre-critical philosophy of mind. I do this for two reasons: his pre-critical views are interesting in their own right, and understanding them provides vital context for understanding his critical position.

Kant’s position can be summarized concisely. His pre-critical philosophy of mind rests on two novel claims about action and change and two metaphysical assumptions. According to the first novel claim, it is possible for bodies to act without causing motion, and it is possible for bodies to be acted upon without being caused to move. According to the second, every change in our world involves the exercise of a "transeunt inner force", an externally-directed force that affects the inner states of the substance it is directed against. According to Kant’s pre-critical metaphysics, bodies are substances composed of monads and souls are immaterial simple substances. Using all four of these claims, Kant concluded that there is no mystery about how an immaterial souls can cause change in material bodies: like all the other cases of action in our world, a soul acts on a body by exerting its force in a manner that causes transeunt inner change in the bodies’ constituent monads. He gave a similar account of the possibility of bodies acting on souls: a body acts on a soul by exerting force on the soul in a manner that causes the souls to undergo transeunt inner change.

It is the task the remainder of Part I of the tour to explain this Kantian solution to the mind/body problem. I have aimed this analysis—which requires some close textual exegesis and background information about Kant’s pre-critical metaphysics—at interested non-specialists, but I hope that this treatment of an important and much-neglected topic will also be of interest to specialists.

SECTION 1: INTRODUCTION AND BACKGROUND

I will begin by surveying several topics in Kant’s pre-critical philosophy that are essential for understanding Kant’s solution to the mind/body problem in Kant’s first philosophical publication, Thoughts on the True Estimation of Living Forces (1747). These topics are real possibility, change, and interaction.

Kant used the notion of real possibility to frame many questions about the philosophy of mind. For example, he sought to explain how the action of matter on mind was really, and not merely logically, possible. His notion of change was centrally relevant to many of the doctrines that constitute his solution to the mind/body problem, for example his doctrine that every change involves the exercise of a transeunt or outwardly-directed force.

For similar reasons, it is important to understand Kant’s general account of interaction: Kant’s views on mind/body interaction can only be understood in terms of his wider account. Kant believed that the specific interactions that are possible in our world are determined by the principles that govern the systematic unity of our world. He also maintained that those principles govern the specific way that substances’ force is exerted in our world, namely gradually or a bit by bit over time.

A lifelong concern with real possibility

In addition to metaphysics, Kant’s early writings focused on the sciences of mechanics, dynamics, and cosmology. He discussed space, time, and motion and problems such as defining force and reconciling mechanism and teleology. This led him to consider what he later called "real possibility". For example, he was concerned with the real—and not merely logical—possibility of the action of matter by means of forces. Kant also discussed quite general questions of real possibility, for example the real possibility of change and coexistence.

This interest continued in the critical period. In the Metaphysical Foundations of Natural Science (1786), Kant worked on many of the same scientific questions. In the Critique of Pure Reason (1781/7), he gave a new account of the real possibility of coexistence and change. In both periods, and indeed throughout his life, Kant sought a method of philosophy that could explain real possibility.

Kant’s notion of real possibility is notoriously difficult to interpret, and in the pre-critical period Kant’s understanding of real possibility was relatively undeveloped. His early use of the idea may have been influenced by Leibniz’s distinction between the principles of non-contradiction and sufficient reason, which suggested that philosophy must strive to do more than demonstrate that a concept is free from contradiction. Kant maintained that explaining the genuine or real possibility of a concept required showing that the concept has a ground in actual existence. For example, one way to show that a concept is really possible is to experience an instance of that concept. This illustrates that, despite the possible influence of Leibniz’s use of the principle of sufficient reason, Kant’s notion of real possibility was resolutely anti-Leibnizian: Kant rejected the Leibnizian idea that possibility can be defined independently of the actual, and independently of all knowledge of the actual.

Kant’s interest in real possibility went beyond empirical knowledge of a concept’s instances. He was also interested in the real possibility of concepts whose instances either do not exist at a certain time or that exist but cannot be the objects of empirical knowledge. In the critical period, Kant’s strategy for handling these cases was to connect grounds of real possibility with the conditions of the possibility of experience. In the pre-critical period, however, Kant had no clear method for handling cases of real possibility that went beyond what can be found in experience, although he did use one crucial resource, the concept of God, the instance of which he believed existed but was not an object of experience. God’s existence provided Kant with a model for judging that a concept was really possible even though its object was not found in experience. As I show below, the concept of God played a prominent role in Kant’s pre-critical philosophy of mind.

Pre-critical dogmatism and the invention of primary forces

Kant’s lifelong concern with real possibility was an important continuity between his pre-critical and critical works. In a study of Kant’s pre-critical philosophy of mind, however, it is important to emphasize how Kant’s early understanding of the mind/body problem was not critical. There was an important difference between Kant’s critical and pre-critical discussions of real possibility. To use a critical distinction, Kant’s early arguments were connected with the possibility of the existence of things (e.g., of objects coexisting or changing), his later arguments with the possibility of our experience of those things. In the critical period, Kant did not attempt to explain the possibility of objects simpliciter; one of his central critical claims was precisely that we can have no knowledge of the real possibility of an object considered apart from the context of our experience. In the Third Analogy in the Critique of Pure Reason, Kant argued that our perception of objects coexisting in space presupposes the concept of dynamic interaction—the formal intuition of space presupposes the concept of interaction in a transcendental sense. By contrast, in Living Forces, Kant discussed the real possibility of mind/body interaction in what he would later call a transcendentally realist sense.

One of the conclusions of the Third Analogy has special relevance for a study of Kant’s pre-critical philosophy of mind. The Analogy was meant to establish the real possibility of dynamic interaction in (roughly) this sense: This possibility is established a priori because Kant showed that the concept is a presupposition of our perception of bodies in space. In the Discipline of Pure Reason, Kant cautioned that "it is only possible for our reason to use the conditions of possible experience as the conditions of the possibility of things, but it is by no means possible for it as it were to create new ones, independent of these conditions" (A771/B799). Kant concluded, in part, that we cannot hope to demonstrate the real possibility of several logical possibilities, including three that are important to the philosophy of mind: "new original forces" (A770/B798), a substance "which would be present in space without impenetrability" (A770/B798), and the doctrine that the soul is a simple substance (A771/B799).

Each of these topics will be highlighted later in this field guide, as will Kant’s changing claims about the possibility of demonstrating their real possibility. As I show below, Kant’s pre-critical solution to the mind/body problem attempted to demonstrate these same real possibilities.


SECTION 2: KANT’S PRE-CRITICAL SOLUTION TO THE MIND/BODY PROBLEM

The key to understanding Kant’s pre-critical solution to the mind/body problem is his distinctive pre-critical understanding of motion, which presupposed conceptions of action and change that were considerably broader than those defended by Wolff and other the other members of the German rationalist tradition in which Kant had been schooled. In the first sub-section, I discuss Kant’s opposition to the traditional view of motion that Kant opposed, a position I label the vis motrix view. This provides us with the background necessary to understand Kant’s solution to the mind/body problem, which I provide in the second sub-section.

Vis activa is not vis motrix

At the beginning of Thoughts on the True Estimation of Living Forces, Kant introduced the notion of a body's essential force. On the vis motrix view defended by Wolff and other post-Leibnizian German rationalists, bodies had no essential force, because force was "regarded as something which is communicated from a body entirely from without" (§1; 1:17). Kant maintained that metaphysical arguments proved that bodies possess an essential force, and he criticized Leibniz's successors for adopting the anti-metaphysical methodology of looking "no further than the senses teach" (§1; 1:17).

In sections two and three of Living Forces, Kant criticized the vis motrix view on two separate grounds. In section two, he argued that Leibniz's empirically-minded successors "would have been well advised to follow [Leibniz] in his metaphysical doctrines" (§2; 1:18). Instead, they used empirical observations to give Leibniz's metaphysical notion of force a more "definite" definition, but in doing this, Kant argued, they made the notion of force devoid of explanatory content.

In the next section, section three, Kant's reasoning is not entirely clear. He suggested that the vis motrix view is not just explanatorily empty, but also involves fundamental metaphysical incoherence. Kant believed that there was something incoherent in conceiving of force as something that a body has only when it is in motion. He also introduced here his vitally important criticism of the idea that changes in motion are the only effects of the exertion of force. As I discussed above, Kant's solution to the mind/body problem affirmed that bodies exert a force that can have as its effect both changes in motion and the production of mental representations.

Kant began Living Forces "by defining certain metaphysical concepts bearing on force in bodies in general" (§1; 1:17). His definitions turned on a series of distinctions: between essential and non-essential force, between force that is internally directed and force that is outwardly directed, and between active and moving force. Like Leibniz, and in contrast to those post-Leibnizian philosophers who accepted the vis motrix view, Kant affirmed that "every body has a force essential to it" (§1; 1:17). Wolff and the other proponents of the vis motrix view held that force is inessential because it "is communicated to a body entirely from without" and it is something "in which the body does not participate when in a state of rest" (§1; 1:17). One can thus distinguish two claims that Kant made about a body's essential force. First, if a body has a force essentially, this cannot be because the force is communicated to it "entirely from without." To say that a body has a force essentially is to make a metaphysical claim that the source of that force is the body itself. Second, if a body has a force essentially, it always has the force, even when the body is at rest.

It is important to emphasize that, despite his agreement with Leibniz that every body has a force essential to it, Kant's own position was profoundly anti-Leibnizian. Kant maintained that every change involves the exertion of a transeunt or externally-directed force. This contradicted Leibniz's doctrine of pre-established harmony, according to which each substance acted only through self-affection or by the exercise of internally-directed forces.

What Kant admired most about Leibniz was Leibniz's commitment to a metaphysical methodology: Leibniz believed that a metaphysical examination of body and force was necessary to understand action, change, and motion. This methodology contrasted sharply with that adopted by the defenders of the vis motrix view, whom Kant considered to be overly enthusiastic about the role of experience in philosophy. Kant believed that, as Leibniz had taught, a metaphysical examination would show that every body has a force essential to it and therefore that the vis motrix view was incorrect. It must have seemed particularly ironic to him, therefore, that Leibniz's own successors adopted an anti-metaphysical stance that led them to consider force by "look[ing] no further than the senses teach"( §1; 1:17).

Drawing only on what they found in experience, the defenders of the vis motrix view asserted that "a body which is in motion has a force...[for example when it] overcomes hindrances, bends springs, and displaces masses" (§1; 1:17). They held that a body has force only when it causes changes of motion in other bodies, for example when a weight bends a spring or displaces a mass. Bodies that do not cause changes in motion, for example bodies at rest, were held to possess no force. This view sounds odd to modern ears, but the notions of kinetic energy and work were undeveloped in the first half of the eighteenth century. This is why the defenders of vis motrix did not accept that a body at rest—for example a compressed spring or a ball at the top of an incline—can possess potential energy. In this, the defenders of vis motrix were in step with the mainstream of seventeenth century mechanics, according to which a body at rest acquired a force only when it was set into motion after colliding with a moving body. Even though he defended the contrary view that bodies have force essentially, Kant was forced to admit that the only available model of essential force, Aristotle's notion of an entelechy, was obscure and that nobody had yet "understood this mysterious teaching" (§1; 1:17).

Leibniz attributed to substance vis activa, an internal, active force modeled after the Aristotelian entelechy. Kant praised Leibniz—"to whom human reason owes so great a debt"—for teaching that "in body there inheres a force which is essential to it, and which indeed belongs to it prior to its extension" (§1; 1:17). Leibniz held that a monad acts by exerting its vis activa on itself in a manner that causes its own inner perceptions to change. This idea was central to Leibniz's pre-established harmony, according to which this is the only way that monads can act. Since Kant's monadism embraced physical influx between monads, he would not have praised Leibniz for this. Rather, Kant was indebted both to the idea of the essential force of a body and to a presupposition of Leibniz's conception of active force, namely that not all action involves motion. I explain this last below, when I address Kant's confusing argument in Living Forces section three that the vis motrix view is metaphysically incoherent. I will first discuss a criticism that is easier to understand, namely Kant's claim in Living Forces section two that the vis motrix view cannot be employed as an explanation for the cause of motion.

Kant's first criticism: the vis motrix view cannot explain the cause of motion (Living Forces, part I, §2)

Kant began section two by noting the irony that Leibniz's own followers re-interpreted his notion of an essential active force in a manner that made it accidental:

      Leibniz gave to this force the general title of active force. His successors would have been well advised simply to have followed him in his metaphysical doctrines; but the attempt has been made to define this force more definitely. The force in body, it is said, is a moving force, since we never observe it to generate anything except motion. If it exerts pressure, it is striving towards motion; but the force is only then exercised when the motion is actual" (§2; 1:18)

Their reliance on experience misled the defenders of the vis motrix view in two ways: they were misleadingly caused to believe that the effect of force is always a change of motion (Kant's first point in this passage) and that only bodies in motion possess force (Kant's second point). Kant maintained that stationary bodies possess a force, for example "a sphere which through its weight presses upon the table on which it lies" (§3; 1:18). Nor is it true, Kant argued, that all action involves a change in motion. If this were so, then the sphere on the table would not exert a force or act in any way because it neither moves nor causes the table to move.

Kant's main conclusion in section two was that their reliance on empirical observation led the defenders of the vis motrix view to define force in a way that made them unable to explain the cause of motion. If force is defined as "moving force," Kant argued, then the notion of force is unsuitable for one of its main philosophical tasks, serving as the cause of motion. If a body's force is vis motrix, then the definition of "force" presupposes the very thing that force is supposed to explain.

Kant argued that to say that moving force is the cause of motion is as vacuous as claiming that a vis calorifica is the cause of heat:

      I assert that in professing to secure an answer to the question of the cause of motion, by thus ascribing to body an essential moving force (vis motrix), we practically resort to the same artifice as the Schoolmen, who in their inquiry into the grounds of heat and cold took refuge in a vis calorifica aut frigifaciens." (§2; 1:18)

It was precisely by eschewing Leibniz's metaphysical definition and searching for a more "definite" definition that accorded with common experience that Leibniz's followers squandered the promise of Leibnizian vis activa. Wolff and others did not correctly identify the cause of motion, an error that caused them to misunderstand the nature of motion and of the laws of dynamics and mechanics. To be sure, Kant did not think that the Wolffians had misunderstood the content of those laws; he admitted that the consequences of the error "do not indeed show themselves in mechanics and natural philosophy" (§5; 1:20), which is to say that the defenders of vis motrix could do mechanical calculations correctly. However, Kant concluded that construing force as vis motrix presupposed a false metaphysical understanding of the nature and source of the laws of natural science.

It is important to realize that Kant's comparison of vis motrix to vis califorica went only so far. Even if vis motrix cannot be invoked as the cause of motion, does this really show that Leibniz's followers misconceived the nature of motion? Kant attempted to show this by raising a second problem with the vis motrix view: its supporters' conception of mechanics was based on an untenable conceptual foundation because their notion of force was metaphysically incoherent.

Kant's second criticism: the vis motrix view is metaphysically incoherent (Living Forces, part I, §3)

Kant made this criticism in Living Forces section three, which is, word for word, one of the most obscure passages Kant published. This section's ambitious scope far out-reached its scant length: Kant attempted, in just seven sentences, to discus three examples where the degree of a body's motion is not commensurate to the extent of its action and to provide at least two arguments that the vis motrix view is metaphysically incoherent. The examples used a tenet of the vis motrix view—that a body acts when it overcomes hindrances—to show that "we do not speak correctly if we treat motion as a kind of action, and so ascribe to it a force synonymous with it" (§3, sentence one; 1:18).

Kant concluded that vis motrix is not an "appropriate title" for the force of a body: a body in inertial motion does not act while a body acts greatly at the very moment it is brought to rest (§3, sentence seven; 1:18). One of his examples was meant to show that a body acts as it is brought to rest. Another was meant to show that a body could continue to act while it is stationary. Unfortunately, Kant's discussion of these examples was obscure. In the fifth sentence, he referred to "bodies which act while they are at rest" and gave with no explanation the example of "a sphere which through its weight presses upon the table on which it lies" (§3, sentence five; 1:18). What is clear about this example is that it is a case where a body is not in motion. Kant believed that there is a question about whether or not such a body acts. If it is right to say it acts (as Kant apparently thought, although he gave no reasons for thinking this), Kant asserted that it is wrong to say that is does so in virtue of "striv[ing] to move" (§3, sentence five; 1:18). It is not clear what Kant meant by this, but apparently he thought that the vis motrix view would appeal to this idea and that the idea was absurd. Kant's reasoning remains obscure, but at least his point was clear enough: this example, like his others, was designed to show that, although motion may be one effect of action, it cannot be the only effect, for the degree of action does not always correspond to the degree of motion.

However obscure Kant's arguments, it is clear that he believed that their reliance on common experience led the defenders of vis motrix to erroneously assume that motion is the only effect of action. As he put it, "we should not, therefore, take our title for the force of a substance from that which is not an action" (§3, sentence seven; 1:18).

Fortunately, Kant's overall strategy for arguing against the vis motrix view was fairly clear. The first three sections of Living Forces contained a sustained criticism of the vis motrix view. In summary form, this was Kant's general line of argument:

(1) Leibniz was correct to say that all bodies have an essential force (§1; 1:17);

(2) Metaphysicians following Leibniz fundamentally misunderstood this idea (§2; 1:18), for they assumed wrongly that the only way a body can act is to cause changes in motion in itself or in other bodies (§3; 1:18);

(3) Given this assumption, Leibniz's essential force must be understood as a moving force (i.e., a force through which a body causes motion in itself or in some other body) (§2; 1:18);

(4) Thus interpreted, Leibniz's notion of essential force has no explanatory force: it amounts to saying vacuously that a body moves because it has the force to do so (§2; 1:18);

(5) However, the assumption that the only way a body can act is to move is fundamentally mistaken, for inertial motion is not an action at all and stationary objects can act (§3; 1:18).

The first claim is a point on which Kant agreed with Leibniz. Claims two and three describe positions held by Wolff and the other defenders of the vis motrix view. Claims four and five summarize Kant's two lines of criticism in sections two and three, that the vis motrix view is unexplanatory and that it assumes wrongly that all action is motion. Claim five raises two distinct points, namely that inertial motion is not action and that stationary objects can act. The second case is the most important for this study, for I have demonstrated that Kant's strategy was to show that a body can exert a force whose effect is something other than producing a change of motion.

With Kant’s criticism of the vis motrix view in hand, we are finally prepared to understand his solution to the mind/body problem. This is the task of the third and final section of this part of the field guide.


SECTION 3: KANT’S SOLUTION TO THE MIND/BODY PROBLEM

Change is not just change in motion: Kant’s account of transeunt internal change

In Living Forces section four, Kant discussed the notion of transeunt internal change. Leibniz believed that the only effect of a substance’s force was a change in that substance’s own internal states. Kant’s conception of change was broader than this in two respects: he believed that transeunt or externally-directed force was involved in every change and he believed that force could have external effects including changes of motion.

Kant’s conception of change was also broader than that espoused by Wolff and other defender of the vis motrix view, which held that force may be transeunt but is always external because forces cause changes in motion only. Wolff and other post-Leibnizian German rationalists went astray, Kant argued by giving a specific definition of force as whatever changes a body’s state of rest or motion. Kant posited a broader and more abstract explanation of the effects of force: substance A exerts a force on substance B just in case A’s agency changes the inner states or determinations of B.

Kant outlined this view in Living Forces section four, where he argued specifically that this change provided the sufficient reason for bodies’ motions and changes of motion. I conclude below that this point was crucial for Kant’s goal of showing that a single force could cause both motion in bodies and representations in souls.

The argument of Living Forces §4

Although he did not define it precisely, Kant’s notion of transeunt internal change supported a comprehensive metaphysical explanation of the world. In Living Forces section four, Kant specified the relation between abstract inner change and the motion of bodies. Here is my reconstruction of his line of argument:

    (1) The force of a substance is determined by its transeunt effects (see §1; this was a presupposition Kant shared with the defenders of the vis motrix view);

    (2) These effects are changes in the inner states of other substances (§4, parenthetical remark to sentence two);

    (3) At the first moment of exertion of force, substance A either exerts all its force at once, or it does not (§4, sentence two);

    (4) There would be no motion if all substances always expended their forces on each other at once (§4, sentence three);

    (5) Since we want to explain motion, we must assume that in our world a substance only utilizes a part of its force at the initial moment of exertion (§4, sentence four);

    (6) A substance must utilize all of its force: it must act with all its force and it must have an effect that is commensurate with its force (§1 and §4, sentences five and six);

    (7) The consequences of this exercise of force are experienced by us in the successive series of things, i.e. in time (§4, sentence seven);

    (8) Bodies thus apply their force on other bodies not all at once, but gradually (§4, sentence eight);

    (9) Since each substance that is acted on by a body receives only part of that body’s force, a body cannot act on exactly the same substance in subsequent exertions of its force (§4, sentences nine and ten);

    (10) It follows that substance A must exert its force on different substances at different times (§4, sentence eleven);

    (11) There must exist a ground or sufficient reason why substance A exerts its force on particular substances at different moments (§4, sentence twelve);

    (12) The ground for this is that as A acts successively it changes its position; substance A is in motion relative to the substances on which it acts (§4, sentences twelve, thirteen, and fourteen).

The "nach und nach" thesis

Kant thought that his account of a transeunt force that caused inner change could also explain the motion of bodies. As he put it at the start of section four, "There is, however, nothing easier than to derive the source of that which we call movement from the general concept of active force" (§4; 1:19). Namely, as steps three and four summarize, Kant conceived of transeunt internal change as the source of motion. Motion exists because substances exert force on each other "nach und nach" (§4; 1:19), which in this context means gradually, a little bit at a time. If this were not the case, Kant stated, there would exist no motion.

By the "nach und nach thesis" I mean the claim that our world is one where force is exercised gradually, a little bit at a time. Kant apparently found it obvious that a world where substances expended their force immediately would contain no motion, for he stated this dogmatically. As Kant put it at the end of the first paragraph of section four, if the world were like this, then the exercise of vis activa could be explained without our having to "name the force of bodies" or appeal to the concept of motion (§4; 1:19). Thus Kant believed that, to explain motion, we must assume that the nach und nach thesis is true or that substances in our world exercise their forces gradually over time.

This idea provided Kant with a novel explanation of the source of motion: motion is the effect of a transeunt internal force that is exercised gradually over time, which is to say that motion is caused by the deferment of the exertion of force. If the monadic substances that constitute our world were not able to resist each other’s vis activa, the nach und nach thesis would not obtain and our world would be motionless. Kant thus held that our world is composed of substances that have both an active and a passive power: every substance exerts force on other substances, and each substance resists the force impressed on it by other substances. According to Kant, therefore, the defenders of the vis motrix view were right to think that motion is grounded on substances’ force, but they erred in thinking that the exercise of force cannot cause anything else besides motion.

Living Forces was a dogmatic text based on incomplete philosophical project. Nonetheless, Kant’s first publication was an important work, both because it set forth large elements of Kant’s pre-critical metaphysical system and because —as I have shown elsewhere—here for the first time Kant raised issues and problems that set him on the path towards the critical philosophy. Under no illusions that Kant’s first work was complete or tenable on its own, in the time that remains, I will sketch out as clearly as possible the remainder of Kant’s metaphysical vision, with the goal of making understandable its most important element, Kant’s first solution to the mind/body problem.

First application of Kant’s account of vis activa: All substances in our world are in space (Living Forces, part I, §§7-8)

Here I connect Kant’s notion of transeunt inner change with his explanation of the unity of our world. It followed from the argument of Living Forces section four, I maintain, that our world is unified spatially. From the specific manner in which the nach und nach thesis is realized in our world, an important consequence followed: all of the substances in our world—including souls—are located in space.

In Living Forces sections seven and eight, Kant affirmed the possibility of a plurality of actual worlds. He defined a world as a whole that is not a part of anything else. A world is not itself a substance, but rather is a composite of the substances that constitute its parts. These parts compose a genuine unity in virtue of the way that they relate to each other. Specifically, a world is unified in virtue of the principle of influx that specifies the manner that substances can act on each other. As many actual worlds are possible as are principles of influx; Kant concluded "it is actually possible that God has created many millions of worlds" (§8; 1:22). Each world would consist of a set of substances that are connected together by a different type of influx. Kant sometimes called this type or principle of influx the form or schema of a world.

Kant conceived of two broad categories or types of worlds, each of which contained a different type of substance. First, a world may contain just one substance, namely a solitary substances that is capable of interacting with nothing else. This is the limiting case: a solitary world has a form that makes impossible any influx. Second, there are worlds that contain several finite substances, all of which interact with each other in virtue of a principle of influx. In these populous worlds the nach und nach doctrine either holds true, in which case the world contains motion, or it does not, in which case the world is static and, Kant suggested, is not a spatial world. That our world contains motion implies that it is a world of the second type whose form or schema involves a principle of influx that causes vis activa to be expended successively.

Worlds, for Kant, denoted limits of interaction: no substance in one world can interact with a substance in another world. Solitary substances are, by the nature of the schema of their worlds, incapable of interacting with anything else. Nor can substances in two populous worlds interact: if two worlds’ principles of influx permitted inter-world action, then the worlds would each be parts of a greater unity, not wholes that are parts of nothing else. It follows that each of the spatiotemporal worlds is unified by a schema that allows its substances to interact with each other but not with the substances in any other world.

Kant maintained that each populous world where the nach und nach thesis obtains would have a different type of spatiality. The schema of our world, he believed, is such that in it the nach und nach thesis is realized in a way that causes the substances in it to interact in a three dimensional space where Newton's inverse square law of universal attraction holds true. It is just because our world is one where substances act outside themselves in a certain way that our world has these features. Our world is not unified with the substances in other worlds precisely because those substances possess different forces and exist in a different type of space. In those other worlds, the specific nature of substances' exertion of vis activa makes possible different types of interaction and spatiality.

The argument of §4 extended

Kant's discussion of worlds in sections seven and eight of Living Forces allowed him to conclude that in worlds where the nach und nach thesis obtains, all substances—material and immaterial—are located in space. This allowed him to add three important steps to the argument of section four:

    (13) A substance exists in a certain world just in case it is capable of interacting with the other substances in that world (§7);

    (14) In a world where vis activa is exerted successively, external relations of this kind entail a spatial location (§8);

    (15) Since our world is one where vis activa is exerted successively, it follows that all the substances in our world are located in space.

Step thirteen followed from Kant's definition of a world. Kant took himself to have shown in section four that, in each world where the nach und nach thesis obtains, the world’s schema or form is such that its substances interact via an influx that puts them in spatial relation to each other. To show that all the substances in our world are located in space, Kant required two things: (1) knowledge that our world is one where the nach und nach thesis obtains, and (2) an argument that the nach und nach thesis entails that substances bear spatial relations to each other. The first point Kant considered inductively proven by everyday experience. The second Kant considered himself to have demonstrated in section four. Kant concluded that the ground or sufficient reason why substance A exerts its force on substances B and C at different moments is that that A bears different relations of position and location to those substances. Since the nach und nach thesis requires interaction to be successive, this conclusion guarantees that nach und nach worlds are spatial, i.e. are worlds composed of substances that bear spatial relations to one another. Thus all the substances in our world are located in space.

The crucial point for the purposes of this work is that Kant applied this conclusion to all the substances in our world, material and immaterial alike. In particular, souls, which he considered immaterial substances, are located in space. This proved crucial to Kant's proposed solution to the mind/body problem.

Second application of Kant’s account of vis activa: Kant’s first solution to the mind/body problem (Living Forces, part 1, §§5-6)

Now, finally, I can present and evaluate Kant’s first solution to the mind/body problem. I first argue that Kant understood the traditional mind/body problem to presuppose several false interrelated assumptions, namely that bodies’ force is vis motrix, that bodies act only by causing changes of motion, that bodies can be acted upon only by being moved, and that souls and bodies do not share a common force. Next I discuss why Kant believed that the vis motrix view was incompatible with mind/body interaction; these sub-sections address, respectively, the difficulties with matter acting on mind and the difficulties with mind acting on matter.

All this prepares the way for a discussion of Kant’s positive solution to the mind/body problem. I argue that his account of mind/body interaction can be understood as an application of his account of transeunt inner change. In accordance with the divine schema of our world, both souls and bodies possess a vis activa that is exerted successively, and that has as its effect both the production of motion in bodies and the production of representations in souls. This follows because a condition of being in our world is being located in space, and a substance can be in space only if it is capable of acting on and being acted upon by every other substance in the world.

I criticize Kant’s argument for being dogmatic, for failing to exclude the possibility of an objectionable hylozoism, and for presupposing a metaphysical dualism that is extremely difficult to understand. To summarize one important problem, Kant’s understanding of the divine schema of our world entails that each substance in our world continually exerts an attractive force on every other substance, then there is reason to worry that souls are the same type of simple substances as the monadic constituents of bodies. Although this conclusion would seemingly strengthen Kant’s claim that souls and bodies are capable of interaction, I believe that it may have committed Kant to an odd and possibly objectionable form of materialism, according to which souls are not matter but are of a material nature.

Kant’s understanding of the mind/body problem in 1747

Kant conceived of the mind/body problem as a series of related difficulties with understanding how souls can act on bodies and how bodies can act on souls. In each case, Kant argued, the difficulties arise only if one assumes that vis activa is vis motrix. He titled Living Forces sections five and six "the difficulties regarding the action of body and soul which arise from the view that body has no other force than vis motrix" (§5; 1:19-20) and "the difficulty which similarly arises regarding the action of soul upon body, and how through the introduction of vis activa it can be removed" (§6; 1:20). The first paragraph of section six demonstrates how Kant approached the mind/body problem:

      We meet with a difficulty when the question is raised how the soul is capable of setting matter in motion. Both this and the above difficulties [regarding the action of the body on the soul] vanish, and considerable light is cast upon the nature of physical influence, when the force of matter is viewed not in terms of motion but in terms of those effects in other substances which we are not in a position to define more precisely. For the question whether the soul can cause motions, that is, whether it has a moving force, now takes the altered form, whether its essential force can be determined to an outwardly directed action, that is, whether it is capable of acting on other beings outside itself, and so of producing changes in them. (§6; 1:20)

Kant maintained that the alleged difficulties with mind/body interaction all share several false assumptions: that bodies possess vis motrix only, that a body can act only by causing motions in itself or something else, that a body can be acted upon only by being moved, and that the moving force of bodies is alien to whatever type of force immaterial substances possess. These assumptions generated two main difficulties for understanding mind/body interaction. First, if a body can act only by exerting vis motrix, then a body can act on a soul only if it can cause the soul to move. But, Kant objected, such an explanation would do nothing to explain the characteristic effect of matter on the soul, namely the production of representations. If bodily force is a moving force, he concluded, the body's power to produce mental representations is an unfathomable mystery. The second problem is closely related to the first. If bodies can be acted upon only by being caused to move, then the assumption that the essential force of the soul is not vis motrix (but some unknown power) provided no basis for explaining how souls could act on bodies. For these reasons, he concluded, the vis motrix view entails that the nature and possibility of the mind's action on the body are hermetic puzzles that philosophy will never crack.

Kant believed that the traditional conception of the mind/body problem was wrong on all counts. He believed, first, that both main assumptions were false and, second, that applying his account of vis activa could dissolve all of the alleged problems with the action of the mind on the body and the action of the body on the mind. In a slogan, Kant believed that the crucial question was not whether bodies and souls can move each other, but rather was whether each can affect transeunt internal change on the other.

If one accepts that vis activa is vis motrix, Kant admitted, it is indeed mysterious how "matter can be capable…of generating representations in the soul of man" (§5; 1:20). Here is how he put the problem:

      What, it is claimed, can matter do beyond causing motions? All its force can at most result merely in displacing the soul from its position in space. How is it possible that the force, which can only give rise to motions, should generate representations and ideas? The latter being things of so entirely different an order from motions, it is not conceivable that they should have their origin in a force of that description. (§5; 1:20)

If the vis motrix view was correct, then motion would be the only effect that matter could cause. Kant found it is "paradoxical" to think that something that can cause motions only could "impress certain representations and images on the soul" (6; 1:21). To think that motion could do this, Kant judged, was an inconceivable non sequitur.

Of course, Kant himself denied that motion is the only effect of the exertion of a body’s force. He believed that the primary or essential effect of force was change in a substance's inner states. To be sure, he also maintained that motion may be a secondary effect of the exertion of vis activa; this is the case in those worlds—including our own—whose schema or form determines that vis activa is exerted successively. However, even in worlds where the nach und nach thesis holds true, Kant's position was that matter can exert force without causing any motion, which was what his prized but obscure example of a sphere resting on a table was meant to demonstrate.

Kant held a similar attitude about the alleged mystery of the mind's action on the body. If the vis motrix view is correct, then the action of the mind on matter is just as mysterious as the action of matter on mind. If vis activa were vis motrix, then the mind could act on matter only if it could cause the body to move, but once again this seems impossible because immaterial substances are "things of so entirely different an order from motions" (§5; 1:20).

However, Kant denied that matter can only be acted upon by being moved. According to his monadism, matter is composed of monadic or simple substances. Matter changes, Kant concluded, when and only when a monad's internal states are changed by another monad's vis activa. As he argued in Living Forces section four, the motion that we sometimes observe accompanying change is a secondary phenomena that arises when vis activa is exerted gradually over time. Kant believed that the mind/body problem is dissolved "when the force of matter is viewed not in terms of motion but in terms of those effects in other substances that we are not in a position to define more precisely" (§6; 1:21). Indeed, it was precisely by attempting to define Leibniz's notion of vis activa more precisely that Leibniz's successors generated the difficulties with understanding force, action, change, and mind/body interaction. Kant's own notion of transeunt inner change was designed to turn away from the vis motrix view and recapture the philosophical utility of a vis activa whose activity is understood—in a general sense only—to cause change in a substance's inner states.

I have explained Kant's strategy for solving the mind/body problem, but have as yet neither presented nor evaluated the details of his solution. Uppermost among the questions about Kant’s solution are whether his account of vis activa and transeunt inner change could really account for the body's capacity to cause representations in the mind and explain the real possibility of matter being acted on by an immaterial substance. Kant's line of reasoning continued the extended argument of the opening sections of Living Forces:

    (16) It follows from steps 1-15, that the source of motion is not a moving force;

    (17) Likewise, physical influence does not have its origin in moving forces;

    (18) Physical influence, rather, has its source in the external effects of vis activa;

    (19) The mind/body problem has its source in the mistaken belief that bodies have an essential moving force that is of a different order from whatever force spiritual substances possess;

    (20) Since all substances in our world possess vis activa that is exerted in accordance with our world's schema, the problem of causal interaction between minds and bodies is to be solved in precisely the same way as the problem of causal interaction between bodies;

    (21) Namely, the possibility of causal interaction between minds and bodies will be proven if one can show that souls, like bodies, are capable of acting on things outside themselves;

    (22) Since each substance in our world is present in space (step 15), it follows that each soul is present in space;

    (23) Since a necessary condition of being present in space is acting outside oneself (steps 13 and 14), it follows that the soul is capable of acting on things outside itself;

    (24) Indeed, since both bodies and souls are present in space, it follows that, in accordance with the schema of our world, each type of substance must be capable of changing the inner states of the other type;

    (25) Since the motion of bodies is a secondary effect of changes of this sort, it follows that a soul is capable of causing a body to move by changing the inner states of the monadic substances of which that body is composed; and

    (26) Since each soul is a monad whose inner state is "the compound of all its representations" (§6; 1:21), it follows that a body's capacity to change the inner state of a soul implies a capacity to cause representations in that soul.

Kant argued that the real possibility of interaction between our bodies and our souls is guaranteed by the way that, in accordance with the divine schema, the nach und nach thesis holds true in our world. Kant's argument rests on two claims. Against the vis motrix view, Kant argued, it is possible for bodies to act without causing motion, and it is possible for bodies to be acted upon without being caused to move. Kant's example of a sphere sitting on a table provided him with a concrete model of this: the sphere acts on the table in a way that involves no motion, for the weight of the sphere presses down on the table even when the sphere is at rest. Kant's deep point about change was that this case is no different from those where change is accompanied with motion: at the most fundamental level, all change is change in a monadic substance's inner state. When the internal states of the monadic constituents of a body change in this manner, the body often moves, although, as the sphere example was meant to demonstrate, this is not always the case. Thus there is no mystery about how an immaterial soul could cause a body to change motion: like all the other cases of action in our world, a soul acts on a body by exerting its vis activa in a manner that causes transeunt inner change in the bodies’ constituent monads.

The possibility of a body acting on a soul can of course be explained in exactly the same manner: a body acts on a soul by exerting vis activa on the soul in a manner that causes the souls to undergo transeunt inner change. Kant's solution gave him what he thought he needed to explain specifically why it is possible for bodies to act on souls in a way that causes changes in souls' representations. Unfortunately, his argument dogmatically presupposed that "the whole inner state of a soul" is nothing but a manifold of representation (§6; 1:21). Although rationalist metaphysicians had long held similar views, Kant did nothing to defend or explain this claim. He could perhaps be excused for not defending a philosophical commonplace of his time, but in this case his silence vexes contemporary interpreters.

One problem is with understanding how Kant distinguished the monads out of which bodies are composed from the monads that are identical with souls. Was his view that the inner states of the former—what he later called "physical monads"—consisted of manifolds of representations? If so, Kant would be hard pressed to avoid the hylozoism he criticized in Leibniz. However, if the inner states of certain monads were not manifolds of representation, would not this require him to justify his claim that the inner state of each soul is simply "the compound of all its representations" (§6; 1:21)? By design, Kant's conception of monadic inner change was bereft of specificity – he described this change as causing "effects in other substances that we are not in a position to define more precisely" (§6; 1:21). Unfortunately, Kant's account of monads themselves was similarly imprecise, an imprecision that would eventually prove fatal to the system of metaphysics he developed in the 1740s, 1750s, and early 1760s.

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