In his first published work, Thoughts on the True Estimation of Living Forces, Kant tried to show that a transeunt force that causes change in substances' internal states could be the cause of both motion in bodies and representations in souls. Kant contrasted his understanding of force to the common vis motrix view, which he considered to be incompatible with mind/body interaction. In this paper, I discuss Kant's rehabilitation of the Leibnizian concept of vis activa and his use of it to solve the mind/body problem. I focus on understanding and evaluating the argument of Living Forces, Part I, sections four through eight.
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 defenders 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 conclusion was crucial for Kant's project of showing that a single force causes both (change of) motion in bodies and (changes of) representations in souls.
The argument of Living Forces §4[i]
Kant's conception of transeunt internal change supported a comprehensive metaphysical explanation of the world. In Living Forces section four, Kant specified the relation between substantial inner change and the motion of bodies. Here is my reconstruction of his line of argument:[ii]
(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.[iii]
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 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.[iv]
Living Forces was a dogmatic text based on an 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[v]. 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)
In this section, 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, two important conclusions followed. The first conclusion was that all of the substances in our world are located in space. The second was that each substance possesses an attractive force that attracts all other substances in accordance with Newton's inverse square law of universal gravitation.
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 later—for example, in the metaphysical works of the mid-1750s—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.[vi] 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. Thus 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. 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.[vii]
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)
In this section, I 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 ground for my discussion of Kant's own 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. One problem with Kant's dualism is this: since 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, 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 unattractive 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 the young Kant understood 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 notvis 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 ofvis 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.[viii]
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)[ix]
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).[x] 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.[xi] 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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