The last of his kind? Gottfried Ploucquet’s occasionalism and the grounding of sense-perception

ABSTRACT

Sufficiently grounding the origin of sense-perceptions in the mind is an issue that has concerned philosophers for a long time, and remains an issue even today. In eighteenth-century Germany prior to the publication of Kant’s Critical philosophy, the two main competing theories to causally ground sense-perceptions were pre-established harmony and physical influx, the latter of which ultimately carried the day. A third option had been around in the seventeenth century: occasionalism. However, historians of philosophy believe this option to have entirely disappeared in the eighteenth century. I will show that this is not the case. In this paper, I focus on one influential German occasionalist: Gottfried Ploucquet (1716–90). Ploucquet not only criticizes Leibniz’s theory of pre-established harmony for providing an ultimately ungrounded, subjective, and arbitrary account of the origin of sense-perceptions, but also presents his own daring alternative: a representationalist-occasionalist theory that locates the objective ground of sense-perceptions in the divine mind.

KEYWORDS:

• Gottfried Ploucquet
• occasionalism
• grounding
• causation
• mind–body problem

1. Introduction

It is a well-known fact that the eighteenth-century German debate about causation – until it by and large collapsed beginning with the publication of Kant’s Critique of Pure Reason (1781) – revolved around two causal theories: pre-established harmony and physical influx. Among other things, these theories were employed to provide an answer to the notoriously difficult problem of the interaction between the mind and the body. While pre-established harmony dominated at the beginning of the century, the theory of physical influx ultimately carried the day (Erdmann, Knutzen und seine Zeit, 55–83; Fabian, Geschichte des Leib-Seele Problems, 224–230; Watkins, “From Preestablished Harmony to Physical Influx”; Watkins, Kant and Causality, 23–100). Philosophers like Georg Bernhard Bilfinger (1693–1750), and Alexander Gottlieb Baumgarten (1714–62) sided with pre-established harmony. Philosophers like Martin Knutzen (1713–51) and Johann Peter Reusch (1691–1758) endorsed physical influx. The fate of a third causal theory, a force to be reckoned with in the seventeenth century, however, has been strangely overlooked: what happened to occasionalism in the eighteenth-century German causation debate?

Occasionalism is the theory that in its most forceful form maintains that secondary causes, both finite minds and bodies, lack any kind of causal power whatsoever. Rather, they are mere occasions that prompt the only truly efficient cause, the first cause or God, to act. Most famously, this theory has been defended by the French Oratorian Nicolas Malebranche (1638–1715). The dissemination of occasionalism in France, the Netherlands, and the United Kingdom is by now well-established. Names such as Géraud de Cordemy (1626–84), and Louis de la Forge (1632–66); Arnold Geulincx (1624–69); and George Berkeley (1685–1753) are merely the tip of the iceberg (e.g. Ablondi, Gerauld de Cordemoy; Nadler, Occasionalism; Schmaltz Early Modern Cartesianisms; Platt, One True Cause; Downing, “Occasionalism and Strict Mechanism”). Only relatively recently has the dissemination of occasionalism in seventeenth-century Germany – the case of Johann Christoph Sturm (1635–1703) – been noted (Sangiacomo, “Teleology and the Evolution of Natural Philosophy”; Sangiacomo, “Sturm’s Natural Philosophy”; Henkel, “Mechanism, Occasionalism, and Final Causes in Sturm’s Physics”). However, the role of occasionalism in eighteenth-century Germany remains largely unknown. This has left historians of philosophy wondering. Corey Dyck, for instance, thinks that “it is a peculiar, and as yet unexplained, feature of the German discussion that no widely influential proponent of the occasionalist system emerged” (“Power, Harmony, and Freedom”, 10f).

In this paper, I will fill this gap in existing scholarship by analysing the case of one influential albeit entirely forgotten eighteenth-century German occasionalist: Gottfried Ploucquet (1716–90). While Ploucquet is aware that occasionalism is a global account of causation, he also believes that it provides a solution to a notoriously difficult problem: body–mind interaction (in this direction) and the origin of sense-perceptions. According to Ploucquet, sense-perceptions require an objective (non-arbitrary) metaphysical foundation. Either (1) they originate within ourselves (that is, they result from our own mind’s activity), or (2) they are caused by some external principle. Ploucquet argues against (1), which he equates with the theory of pre-established harmony, because it does not square with the fact that we are not in charge of our own sense-perceptions, and because such an account would not provide an objective but rather a subjective, arbitrary ground of sense-perceptions. An external principle causing sense-perceptions could either be (2a) finite (bodies or minds) or (2b) infinite, i.e. God. Ploucquet’s ontology, however, reduces bodies to mere passive phenomena (rather than substances). What’s more, he finds any kind of causal influx between substances (minds) implausible for reasons similar to Leibniz’s. In ruling out 2a, i.e. physical influx, Ploucquet is therefore left with 2b, i.e. occasionalism and, indeed, for him, only God, i.e. the realm of the divine mind, can serve as the common and objective ground for our experience of the external world.¹

In focusing on the problem of the origin of sense-perceptions, I follow Ploucquet’s own philosophical predilection. I will concentrate on Ploucquet’s first metaphysics textbook, the Principles concerning Substances and Phenomena (Principia de substantiis et phænomenis) (1753).² Here, occasionalism features prominently in chapters ten, eleven, and eighteen dedicated to the interaction between substances, the origin of sense-perceptions and the interaction between the mind and the body, respectively.

Ploucquet was a university professor for logic and metaphysics at the University of Tübingen since 1750. He won the 1747 essay prize competition of the Berlin Academy together with Johann Heinrich Gottlieb Justi (1717–71), objected to Kant’s The Only Possible Argument in Support of a Demonstration of the Existence of God (1763), and prepared the master’s theses defended by no one else but Hegel and Hölderlin in 1790 (Neumann, “Ploucquet und die Monadologie”, 29; Klemme and Kuehn, Dictionary of Eighteenth-Century German Philosophers, 1242–1244; Franz and Pozzo, “Ploucquets Inauguralthesen zur Metaphysik (1790)”).

The existing literature on Ploucquet is scarce and hardly accessible to the wider audience. The only two book-length studies on Ploucquet are more than a hundred years old, and despite their merits both Bornstein (Ploucquet’s Erkenntnislehre und Metaphysik) and Aner (Gottfried Ploucquet’s Leben und Lehre) present Ploucquet’s philosophy in rather general terms. Ploucquet’s role in the history of aesthetics and psychology has been dealt with – at the end of the nineteenth century and the beginning of the twentieth century – very briefly by Sommer (Geschichte der deutschen Psychologie und Aesthetik, 74–88) and Dessoir (Geschichte der neueren deutschen Psychologie, 165–168). Franz has attended to Ploucquet’s role in logic (“Exkurs zu Ploucquets Logik”). Ploucquet’s indebtedness to Malebranche has been sketched by Pozzo (“Ploucquet und Kant über Malebranche”), but only Specht (“L’Occasionalismo in Germania”) deserves the credit of having studied Ploucquet’s role in the German reception of occasionalism. To the best of my knowledge, my article is the first detailed account of Ploucquet’s occasionalism and the first one in English at that.

The structure of this article is as follows: I will first present the metaphysical foundations of Ploucquet’s occasionalism, that is, his ontology (Section 2). I will then show how Ploucquet himself framed the problem of body–mind interaction and the origin of sense-perception, and why he thinks that both physical influx and pre-established harmony are philosophically insufficient (Section 3). I will then critically examine Ploucquet’s representationalist account of occasionalism itself as a solution to the mind–body problem and, more specifically, as an attempt to properly ground sense-perceptions (Section 4). I will conclude with some more global considerations and I will return to the issue of determining Ploucquet’s place in the wider debate about causation in eighteenth-century Germany (Section 5).

2. Ploucquet’s philosophical foundations

Ploucquet’s ontology consists of three kinds of entities: (1) the infinite substance, or God; (2) mind-like finite substances essentially characterized by a principle to manifest themselves; and (3) phenomena. While the human soul qualifies as a substance (Principles, §124, 64f), bodies are not substances. Ploucquet reduces them in a rather Leibnizian manner to the status of mere phenomena (Principles, §§27, 73, 12, 36). In a Cartesian manner, however, Ploucquet maintains that:

No one in possession of reason [rationis compos] can doubt their own existence. In fact, whoever thinks proves eo ipso their existence, since to do something [operari] and to be nothing or to do nothing cannot stay together. Therefore, this is the first truth which everyone knows a posteriori and intuitively [intuitivè]. I think, or I am something that thinks.

(Principles, §1, 1f)³

The reasoning is clearly reminiscent of Descartes’ Second Meditation in that thinking is considered as a reliable proof of the existence of a thinking substance. Indeed, Ploucquet points out that my experience of my act of thinking makes it impossible to be in doubt about its facticity.

What grounds a substance’s activity of thinking must, according to Ploucquet, be an active principle proper to the thinking substance itself:

Every substance is an active principle. But what is acting? I answer that acting viewed in its first source can be nothing other than to express something in oneself, or to form some image, or to be manifest to oneself. Hence, every substance is manifest to itself.

(Principles, §20, 8)

Ploucquet spells out this active principle in terms of a principle to manifest oneself, a principle of mental presence so to speak. This principle has sometimes been understood in terms of self-consciousness, and justifiably so (see Bornstein, Ploucquets Erkenntnislehre und Metaphysik, 43; Dessoir, Geschichte der neueren deutschen Psychologie, 166). It is clear to Ploucquet that such an active principle must be intrinsic to the substance itself rather than extrinsic as it seems obvious from every-day experience that “someone else does not think in me, but I am thinking” (Principles, §470, 301). The very act of thinking reveals that we are dealing with a kind of self-causation: I am causing my own thoughts. Therefore, the principle grounding my activity of thinking must be in me rather than in someone or something else. The principle of self-manifestation is the one principle defining substances – Ploucquet uses the term “uniprincipialis” (Principia, §23, 10).

Moreover, Ploucquet follows Leibniz in determining substances as “simple and indivisible” (simplex & indivisibilis; Principles, §23, 10) and “one” (unum quid; Principles, §25, 10). Since bodies are extended, they are divisible or reducible to their parts (Principles, §76, 37). Since they are divisible or reducible to parts, they are pluralities of unities (Principles, §22, 9; §33, 33). Bodies are not substances but phenomena grounded in mind-like substances.

3. The mind–body problem and the grounding of sense-perceptions

Now that we are familiar with Ploucquet’s metaphysical foundations, let us turn to the mind–body problem and the origin of sense-perceptions. Ploucquet makes it clear that the mind–body problem consists of two specific parts: (i) the mind–body problem (in this direction), and (ii) the body–mind problem (in this direction). While the former amounts to the problem of the origin of motions in the body, the latter amounts to the problem of the origin of sense-perceptions in the mind (Principles, §457).⁴ The problem of the origin of sense-perceptions is, however, the one far more important to Ploucquet. He frames it as follows:

It is necessary that it is understood what the objective origin [origo objectiva] of such a phenomenon [corporeal phenomena] is and how the representation of a phenomenon [repræsentatio phænomeni] originates?

(Principles, §203, 118)

Ploucquet also realizes that there is a special problem of grounding sense-perceptions but not for ideas in general. This is because in the case of abstract thinking or contemplation Ploucquet believes that it is obvious that we cause and direct our thinking since we are clearly in control. In the case of sense-perceptions, this is, however, not the case. For instance, when we open a book that we have not read before, the input conveyed by our senses is clearly new, and we cannot (continue to) bring about new content outlined in the book when we have closed it (Principles, §166).

According to Ploucquet, the objective ground of our sense-perceptions could either (1) be an internal principle residing in ourselves, such as the mind’s own immanent causal activity, or (2) be an external, mind-independent principle. The latter in turn could be (2a) finite (bodies, or other minds), or (2b) infinite (God). Ploucquet’s discussion of these solutions shows that he equates (1) with Leibnizian pre-established harmony, (2a) with some form of causal influx theory, and (2b) with occasionalism. On what grounds does Ploucquet reject (1) and (2a)?

In opposition to (1), i.e. to Leibniz’s pre-established harmony, Ploucquet argues that substances cannot contain the sufficient ground for the occurrence of their own (sense-)perceptions (of other beings). We will see why in a moment.

In the Monadology, Leibniz had made it clear that not only do substances (or monads, in his terminology) not interact with other substances (or monads), he also pointed out that the change which occurs within finite substances qua created (and hence subject to change) (Monadology, §10), results from an “internal principle” (Monadology §11; Philosophical Essays, 214). Changes in substances representing “a multitude in the unity” of the substance are called perceptions (Monadology §14; Philosophical Essays, 214). Sensations (or sense-perceptions), in turn, are defined as “more distinct” perceptions in more developed substances (Monadology, §19; Philosophical Essays, 215).

Ploucquet objects to Leibniz’s line of reasoning that sense-perceptions in their variety and complexity exceed what a substance could bring about by itself. They exceed what I qua finite substance could bring about. Therefore, they must be brought about by a principle external to me:

Whatever does not flow from my [own] manifestation, or from a representative principle that is in me has its ground [rationem] outside of my manifestative principle [principium manifestativum]. Many phenomena in the world, which is called corporeal, do not flow from my [own] manifestation. The minor [premise] of this syllogism is apparent, since no reason [ratio] can be thought out why I should refer external phenomena to my egoity [egoitatem]. … Hence such phenomena do not pertain to the form of my egoity [egoitatis] or my internal principle, or my soul. … Since, if more were to follow from my own principle, [then] the effect would be greater than the cause; something would follow from a principle that is not intelligible from [that principle] itself. But since external phenomena and ideas excited by them contain in themselves more perfection or greater composition and distinguishability [distinguibilitas] than the likes of which I could form by means of all the power of ingenuity and by means of meditations applied to the most extreme, it is most evident that such ideas do not procced from my [own] principle.

(Principles, §167, 94f)

Sense-perceptions of the external world show a great degree of perfection, complexity, and distinctness. According to Ploucquet, it is a hardly credible story to maintain – as Leibniz would – that these originate from the simple substance’s own representative force. The characteristics of sense-perceptions exceed by far what could be grounded in a substance’s own internal active principle. Ultimately, Leibniz’s account seems to Ploucquet to violate the causal containment axiom according to which whatever is contained in an effect has to have been contained in its producing cause.⁵ Due to their richness, sense-perceptions cannot be contained, not even in nuce in the perceiving substance itself. Differently put, insofar as causes render intelligible their effects, Leibniz’s story would seem hardly plausible to Ploucquet.

He adds to this that if sensations or sense-perceptions in more developed substances like us were due to an internal principle with which we were essentially endowed or identical to, then we should be in charge of our own sense-perceptions. This, however, is not the case. To strengthen this point, Ploucquet adduces a vivid example:

When I am tortured by hunger, thirst, pains, and other perceptions, I cannot make it such that other representations coinciding with these come about – those I have through satiated hunger, quenched thirst, numbed pains. On the other hand, when I do have certain representations, when I am sitting in the dark, when I do not perceive the scent of flowers etc., I can by no means make it such that that which I desire comes about, that I see light that I smell flowers.

(Principles, §170, 97)

Experience, for Ploucquet, establishes that I do not actively bring about my own sense-perceptions. Rather, they are something to which I am passively subjected. Undergoing sense-experiences shows precisely that I am not in control. However, or so Ploucquet thinks, were the Leibnizian account correct, I would need to be in charge of my own sense-perceptions. I would be able to alter them at will.

It is interesting to see that while Leibniz’s theory – pre-established harmony – was usually criticized for being determinist, and abolishing free will, Ploucquet is convinced that the opposite is true. Pre-established harmony proves too much. It goes so far as to ascribe voluntary control over sense-perceptions.⁶ This, in turn, strikes Ploucquet again as providing a subjective and arbitrary rather than an objective ground of sense-perceptions. If I exert voluntary control over my sense-perceptions, why would they coincide with those of other people which they regularly do? We should note, however, that Ploucquet does not produce a knock-down argument. At best, he shifts the burden of proof prompting the Leibnizian to better explain how sense-perceptions really originate within the system of pre-established harmony. At worst, his case rests on a misunderstanding of Leibniz’s position. This is because, on the one hand, he might ascribe to Leibniz a too narrow conception of the working dynamics of a simple substance’s (or monad’s) active principle. The more complex the operations of this principle, the more complex and diverse the mental states that this principle could bring about, and hence sufficiently ground. On the other hand, as far as I know, Leibniz never explicitly says that a substance’s principle of activity or representative force entails voluntary control.⁷ Also, the fact that God arranged simple substances in a harmonious way from the outset of this world and the fact that the relations between substances are wisely pre-established by God could function as the objective ground of sense-perceptions. But be that as it may.

In opposition to (2a), that is, physical influx theory, Ploucquet makes it clear that insofar as transeunt causation is concerned, substances cannot act on one another:

With regard to existence, one substance is independent from another finite substance, and by means of its own powers [viribus] flowing from its own proper source, it neither alters realities in another substance, because from its own manifestation does not flow the manifestation of another, nor does the positing [positio] of one substance bring about the actual interaction with another substance.

(Principles, §71, 35)

Not only are substances beings that exist independently from other beings (except God) such that positing one substance does not logically entail the positing of another substance, but they do not causally communicate with one another, either. The former idea is perhaps taken straight from Descartes’ Principles of Philosophy (1644) (Part I, §51) although Ploucquet does not say so explicitly. The latter idea in turn seems to be inspired by Leibniz’s Monadology (1714):

There is also 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.

(Philosophical Essays, 213f)

Ploucquet clearly read the Monadology as he explicitly refers to it (Principles, §58). He agrees with Leibniz that there is no transeunt causation between monads or (simple) substances. However, according to Ploucquet, this is based on the idea that substances are essentially defined by one principle only and that this is the principle of self-manifestation. Self-manifestation as self-reflexive in turn does not entail any kind of manifestation in another. Neither the very existence of substances nor their essential characteristic provides the sufficient ground for inter-substantial causation. But if there can indeed be no inter-substantial causation between finite substances, how could another (finite) substance be the objective ground of my sense-perceptions? In Ploucquet’s eyes, the system of physical influx, too, remains insufficient.

4. Ploucquet’s representationalist occasionalism

Ploucquet’s rejection of both real transeunt causation (endorsed by the system of physical influx) and the kind of intra-substantial causation endorsed by the system of pre-established harmony to some extent already elucidate his acceptance of occasionalism as the last option to provide an objective ground for the origin of sense-perceptions. Indeed, Ploucquet hints early on that this is the position he will defend:

Everything that happens in a finite substance depends originally on God, and God can excite in multiple ways new perceptions in finite substances. Therefore, passivity is an attribute of the finite substance.

(Principles, §70, 35)⁸

Ploucquet points in the direction of the infinite substance to ground the occurrence of sense-perceptions. As to the passivity of finite substances to which Ploucquet alludes, I read him (in light of the context) as referring to the issue of transeunt causation: Substances do not (causally) ground effects occurring in other substances but God does.⁹ Therefore, this does not contradict substances’ active principle which Ploucquet introduced earlier as this is confined to substances’ immanent actions, i.e. in particular, thinking. Ploucquet makes it clear that God is the ground of the existence of finite substances:

Finite things exist through the action of God, since they do not have so much reality [tantum realitatis] that existence flows necessarily from there [i.e. from them], nor can they be produced by anything else.

(Principles, §187, 108)

It is precisely the finitude of finite substances that shows that they need a ground independent of themselves as a basis of their existence. Thinking of Descartes and perhaps even Anselm of Canterbury, Ploucquet believed that the essence of finite substances does not entail their existence. Since the existence of one finite substance does not entail the existence of another finite substance, finite substances are not produced by other finite substances. Rather their existence is contingent upon God.

While the vast majority of early modern authors surely agree that God as the necessary being grounds the existence of finite contingent beings, Ploucquet spells this out in terms of God’s so-called real vision (visio realis):

An existing finite being is therefore the effect of God’s real representation [realis Dei repræsentationis] which He has of an existing being as such. God, who sees the idea of a self-manifesting being insofar as it manifests itself, thereby produces that being through this real vision [visionem realem].

(Principles, §189, 109)

What God does in creating finite beings is to represent them to himself as existing and being self-conscious. Clearly drawing on Malebranche’s insight (expressed in the Search after Truth, LO, 232) that God’s ideas are efficacious, Ploucquet points out that:

The divine ideas infinitely differ from ours. Our ideas are not so efficacious [operosæ] that a real effect obtains outside of them. God’s ideas are the true sources of every existence, and everything that is really understood depends originally on them.

(Principles, §190, 109)

When God has a certain idea and he wishes that this idea obtains objective reality outside his mind, this comes about. Ploucquet is here borrowing to some extent from Malebranche’s Vision in God doctrine. However, he does not commit himself to the details of Malebranche’s account.

Malebranche initially sets forth his Vision in God doctrine in book three, part two of his Search after Truth (Recherche de la Verité). He later attempts to make it clearer in Elucidation Ten to the Search.¹⁰ Malebranche takes pains to explain how we come to know eternal truths, such as that two times two is four, or that the sum of the interior angles of a triangle equals 180 degrees. In order to do justice to the fact that these truths are true necessarily, immutably, objectively (i.e. represent how things really are) and true intersubjectively (i.e. shared by a multiplicity of human beings and that they are so to speak public domain), Malebranche – following Augustine – places them as ideas in the mind of God:

All our ideas […] must be located in the efficacious substance of the Divinity, which alone is capable of enlightening us, because it alone can affect intelligences.

(Search after Truth, LO, 232)¹¹

Insofar as ideas are in the mind of God who is the only true efficient cause (Search after Truth, 448, 450), they can be called efficacious, too (Search after Truth, 232). In contrast to Augustine, Malebranche later extends the Vision in God doctrine to the case of sense-perceptions. These are the result of our having of a pure, universal idea (say, a triangle) modified by a sensation (say, redness), which is “a modification of our soul, and it is God who causes it [the sensation] in us” (Search after Truth, 234). While ideas represent the (universal) essence of the thing we experience, sensations serve to particularize and individualize it (Search after Truth, 621, 625; Adriaenssen, Representation and Scepticism, 146). While our (imperfect¹²) idea of a thing depends on God’s (perfect) intellect, our sensation of it depends on God’s will bringing about the sensation on the occasion of confrontation with a particular material body.¹³

Let us return to the case of Ploucquet. He explains that even the alleged interaction between finite substances must be explained in terms of God’s efficacious representations:

I conceive the action of a substance on a substance in the following way: God represents to Himself substances as manifest to themselves. Through this act of representation [actum repræsentationis] substances exist. All representations concerning substances as existing are really in God. Therefore, all these real representations are connected among themselves.

(Principia, §200, 114)

We saw earlier that the mere existence of one finite substance entails neither the existence of another finite substance nor any kind of connection between the two. Rather, Ploucquet reasons that the connectedness of substances is grounded in the unity of God’s consciousness. Insofar as substances as representations of God belong to the same mind, i.e. God’s mind, they are connected. The only difference between our own unity of consciousness, and God’s is that the fact that ideas are connected in our mind makes no difference for how things really are outside of our mind. Remember that Ploucquet had emphasized earlier that our mind lacks the kind of real causal efficacy that characterizes God. In addition, Ploucquet shares Malebranche’s opinion that only God truly acts:

Malebranche establishes that it is necessary that there is a real proportion [proportio realis] between God’s will and the effect, and that God’s will is the only true cause. I do not have anything to oppose. But in understanding what is will [voluntas], one finds that the will itself is a kind of representation [species repræsentationis] or that that which originates from representations is called will.

(Principles; §196, 112)

The ultimate root of acting needs to be sought in the nature of the infinite being. … God sees in Himself a finite existing being, and in the finite being he sees Himself through his action.

(Principles, §198, 113)

It is noteworthy that Ploucquet reduces the whole world to the intra-mental spectacle of God’s immanent causal actions. When we believe a substance is acting on another substance what really happens is that God represents to himself that one substance acts on another substance. Given that God is omnipotent, this will also come about. Any apparent transeunt causation, however, is traced back to God’s immanent causation. We will see later that immanent causation is the only kind of causation that Ploucquet retains. In the case of finite minds, it is confined to the mere act of thinking itself.

So far, so good. But more is needed to sufficiently ground sense-perceptions, and up until now, Ploucquet has not said anything sufficiently elucidating to this effect. He has only established the existence of mind-like substances, and their connection, but neither the phenomenological existence of the corporeal world nor the nomological connections obtaining in this world. He is, however, quick to explain that:

Since God represents to Himself infinite phenomena and infinite nexuses manifest or able to manifest [manifestabilia] in such and such a way between infinite phenomena, it happens that God’s real representation of infinite phenomena with their nexuses and relations generates the existence of the corporeal world.

(Principles, §210, 122)

God’s representation of the corporeal world as phenomenal, and his idea of the connectedness of corporeal phenomena is the sufficient ground for the phenomenal existence and coherence of the corporeal world. Even more, when God represents corporeal phenomena as changing for us, they do so change for us. What God represents as remaining stable, remains stable:

That which is more constant in God’s real vision provides the basis for the constant observation of phenomena, and that which in God’s vision is represented as changeable [variabile] effects that it appears as changeable to us.

(Principia, §217, 126)

The case of motion is a further illustration of this principle:

Motion is produced when God really represents to Himself the state of a body following from a previous one and deducible in a very wise manner according to constant and simple laws.

(Principles, §264, 151)

That is, we experience motion when God represents to himself the successive passage of a body from one spatial position to a different one according to certain fixed regularities and when he wants to represent to himself that we become aware of this.

Interestingly, Ploucquet tacitly introduces two levels of representation: (1) God’s representations of either substances’ or corporeal beings’ existence and connectedness, (2) our representations of corporeal beings. Our own representations of the external world turn out to be second-order representations in that God represents to himself that we represent corporeal beings. Our phenomenal experience of the external world is based on God making it such that these experiences appear to us. Obviously, this transforms the corporeal world into a world of appearances, and, hence, smacks of idealism. Sense-perceptions are grounded in God’s immanent-causal, i.e. representative, activity rather than in the transeunt causal action of finite corporeal substances (physical influx), or the immanent causal actions of finite mental substances themselves (pre-established harmony).

Alas, there is one more complication to Ploucquet’s account, and that is that God does not represent to himself sense-perceptions in their complex form but only the simple building blocks from which our complex sense-perceptions are constructed:

God represents to Himself the basis [fundamentum] of all phenomena, or he sees in Himself such ideas out of which can be deduced by means of various compositions, divisions, proportions, and degrees all composed phenomena with their regularities.

(Principle, §214, 124f)

If we wish to add some analytical clarity to this, we might try to reconstruct Ploucquet’s considered view in the following way: The content α of our representation r – which is actually God’s representation R of us having r_α, i.e. R(r_α) – consists of elements A, B, C etc. These elements might be qualified in certain ways: by a coefficient (x), an operation of division (1/y + z) etc. reflecting the physico-psychological properties that very sense-perception (i.e. the representation) has for us: α = {A, xB, C/(y + z), … }. The overall image emerging is R (r_{{A, xB, C/(y + z), … }}). From what Ploucquet says we understand that the suitably qualified simple elements constituting our complex sense-perceptions are put together by means of certain laws of combination. However, Ploucquet does not invest much effort into spelling out either the exact laws governing the composition of a complex sense-perception out of its simple building blocks or what these simple building blocks themselves are. Quite the contrary. He thinks that our sense-perceptions are so complex (compositae) that these can never be found (Principles, §214). What adds to the difficulty of the task is also that Ploucquet believes that the same complex representations or sense-perceptions can be arrived at by different operations of combination (Principles, §215). Even the exact way (ratione speciali) in which God’s real vision of the manifestation of finite principles (manifestationem principiorum finitarum), i.e. the manifestation of finite substances, works exceeds our cognitive faculties as this would require intuitive cognition (cognitione rei … intuitiva) (Principles, §197). This we do not possess. After all, Ploucquet leaves a very interesting approach underqualified. An explanatory gap remains between our complex everyday sense-perceptions, and the simple building blocks and (hardly spelt out) laws of combination from which they arise. What we do know, however, is that God is the only real efficient cause producing our representations of the external world on the occasion of other preceding representations.

On a slightly different note, we might wonder how far Ploucquet’s representationalist occasionalism goes. Importantly, he underscores that in spite of God’s ubiquitous representative activity, mind-like finite substances are immanently causally active insofar as their own abstract thinking is concerned. This means that while all other causal dimensions – body-to-body, body-to-mind, mind-to-body – are explained in occasionalist-representationalist terms, Ploucquet’s occasionalism stops short of being wholesale:

I concede that body has no force to act [vim agendi] in itself, because it is not a self-manifest substance [substantia sui manifestativa], nor can an idea of any action be found [videri] in the idea of body. But I deny that mind [spiritum] is void of a force to act. In the idea of mind [spiritus], I see its manifestation which is a real action. When I think of God, the mind acts: meditating on these arguments, I act. Something else does not think in me, but I think. Every egoity [egoitas] is necessarily an acting thing [aliquid agens]. I do not deny that the source of my existence depends on the divine operation, but this operation itself provides something active. In the real representation of God is contained a representation of a finite active being [entis finiti activi]. Mind [spiritus] cannot be conceived without action, because when action ceases the very existence of the mind is taken away.

(Principles, §470, 301)

We find Ploucquet returning to the Cartesian outset of his philosophy. The self-conscious activity of thinking is really that: an activity. It is constitutive of our being. God provided us with this albeit restricted scope of freedom. The realm of the immanent action of contemplating the world though not acting in it is what cannot be taken away from us without ridding us of our very existence.¹⁴

While Ploucquet shares with his fellow occasionalists the belief that bodies are essentially passive, minds are (immanently) active. This means that the vision in God theory that Ploucquet endorses to account for body–mind interactions and the origin of sense-perceptions does not apply to the realm of abstract thinking. Our finite mind itself is causally responsible for bringing about its contemplative reflections. In this, Ploucquet clearly parts company with Malebranche according to whom the vision in God theory also explains abstract thinking.

5. Concluding remarks

From at least the publication of the Foundations of Theoretical Philosophy (Institutiones philosophiæ theoreticæ) (1772) onwards, Ploucquet clearly dismisses occasionalism and endorses an (albeit slightly naïve) form of physical influx. Somewhat disappointingly, he does not state his reasons for his change of heart. One possibility is that he became increasingly aware of the (monist-)idealist outlook of his philosophy, and of the role that his representationalist occasionalism played in supporting it. We know that Malebranche’s Vision in God doctrine, from which Ploucquet borrows substantially to build his own occasionalist system, had been charged with being idealist by Antoine Arnauld (Adriaenssen, Representation and Scepticism, 155–159). An idealist philosophy might have seemed to Ploucquet to call into question the real existence of the outside world.

Another possibility is that Ploucquet ultimately came to realize that there might be an irresolvable conflict between the conception of finite minds as truly acting and occasionalism as ascribing causal power solely to God. Ploucquet’s main argument against occasionalism – indeed the only one to survive in his later metaphysics textbooks, i.e. the Elements of Contemplative Philosophy (Elementa philosophiæ contemplativæ) (1778) and the Expositions of Theoretical Philosophy (Expositiones philosophiæ theoreticæ) (1782) – is the following:

As Malebranche deprives Minds themselves of an internal principle of acting, the distinguishing feature [character] itself of existence in them is taken away, because an ens Uniprincipalis without an internal striving [nisu] to act coincides with a being [that is] in no way observable, even more, with nothing.

(Foundations 1772, section III, §39, 437)

Ploucquet seems to have realized that when it comes to the activity of minds, Malebranche is not an ally but a foe. Following a more Leibnizian line of reasoning, Ploucquet might have come to believe that a genuine source of activity is required for individuation. Without such a principle, minds as thinking things could not be individuated, or else only extrinsically in the way that motion individuates matter for the Cartesian. Furthermore, Ploucquet might have become aware that his partial occasionalism might consequently lead to wholesale occasionalism and that this would, in turn, square poorly with a vita activa or human freedom.

Nadler, for instance, has shown that Louis de la Forge’s argumentation for partial (physical) occasionalism cannot be contained to the realm of the purely physical. Rather, la Forge faces the undesired consequence that the main argument he uses to establish physical occasionalism, i.e. the argument that conservation is but continuous creation, would also rule out real mind–body as well as intra-mental causation, and hence force him to accept wholesale occasionalism. This would, of course, question the activity of the mind and bring back the daunting challenge of copying with human freedom (Occasionalism, 123–141). For similar reasons, Ploucquet might have ultimately turned his back on occasionalism tout court.

We began our discussion by reflecting on the eighteenth-century German debate about causation. While I do not wish to challenge the picture that the main competitors were indeed the systems of pre-established harmony and physical influx, Ploucquet is the exception that proves the rule.¹⁵ He is also perhaps the most influential eighteenth-century German occasionalist given his importance for future generations of philosophers such as Hegel. In light of the demise and lack of popularity of occasionalism in the eighteenth century, this is perhaps not much.

However, that should not make us miss out on Ploucquet’s creative and almost entirely overlooked representationalist occasionalism. In addition, his constant critique of pre-established harmony together with that of others such as Knutzen and Kant might have tipped the scale towards physical influx. Studying the case of Ploucquet helps us to obtain a more complete picture of our philosophical past.

Acknowledgements

I would like to thank Doina-Cristina Rusu, Andrea Sangiacomo, Matteo Favaretti, Barnaby Hutchins, Hugo Hogenbirk and the anonymous reviewer for BJHP for their constructive feedback on earlier drafts of this paper.

Notes

1 While this might strike the reader as similar to Berkeley’s case for occasionalism, Ploucquet nowhere mentions the Irish Priest. In light of the fact that Ploucquet did not read any English, and that Berkeley’s works were translated into German much later (Bornstein, Ploucquets Erkenntnislehre und Metaphysik, 47; Sommer, Geschichte der deutschen Psychologie und Ästhetik, 77, 87), it seems reasonable to assume that Ploucquet developed his idealist-occasionalist story on the basis of his reading of Descartes, Leibniz, and Malebranche as these are (as we will see) clearly his main sources of inspiration.

2 I will abbreviate it as Principles. While Ploucquet gave up on his occasionalism in his later years, he unambiguously endorses it in the Principles.

3 All translations of Ploucquet’s originally Latin text are my own.

4 The way that Ploucquet (somewhat unsystematically) designed his textbook means that we are referred back to the chapter on the origin of sense-perceptions and the even earlier chapter on the interaction between substances, both of which precede the chapter on mind-body interaction and also by and large provide its solution as well as much of the philosophical argumentation against competing views.

5 Descartes had formulated the causal containment axiom both in the Third Meditation, and in the Second Set of Replies to the Meditations, ax. iv: “there must be at least as much (reality) in the efficient and total cause as in the effect of that total cause” (Third Meditation, CSM II, 28); “Whatever reality or perfection there is in a thing is present either formally or eminently in its first and adequate cause” (Second Set of Replies, CSM II, 116). In the Sixth Meditation, Descartes uses the causal containment axiom to show that the external world must exist as does Ploucquet. I am indebted to Andrea Sangiacomo for pointing out that Descartes and Ploucquet side on this matter.

6 I owe this observation to Matteo Favaretti.

7 What is more, Leibniz would object that the appetites of monads do not produce representational content (such as sense-perceptions) on their own, even if they determine which representational content is pursued. I owe this remark to the anonymous reviewer for BJHP.

8 Ploucquet will later qualify this characterisation of finite substances as passive as much as his rejection of intra-substantial causation. The latter will not be applied to the substance’s own thinking narrowly understood.

9 In contrast to contemporary analytic approaches to grounding, I do not take grounding to be exclusively non-causal. Grounding in early modern philosophy is oftentimes conceived in causal terms.

10 These are the two most prominent though by no means the only places where Malebranche discusses his Vision in God doctrine.

11 Adriaenssen (Representation and Scepticism, 149f) draws attention to the “objective validity” and the “intersubjective validity” that truths obtain in virtue of the fact that they are located in the mind of God.

12 Malebranche points out that “minds do not see the divine substance taken absolutely but only as relative to creatures and to the degree that they can participate in it. What they see in God is very imperfect, whereas God is very perfect” (Search after Truth, LO, 231).

13 Discussion of (the development of) Malebranche’s account of the Vision in God doctrine, his arguments in support of it as well as critique by his contemporaries such as Arnauld and Régis can be found inter alia in Adriaenssen, Representation and Scepticism, 143–159; Pyle, Malebranche, 47–73; Schmaltz, “Malebranche on Ideas and the Vision in God”. Pessin (“Malebranche’s Vision in God”) gives a good overview of the existing scholarly debate.

14 It is interesting to see that Ploucquet believes Malebranche – the prime example of a wholesale occasionalist, an occasionalist across every causal dimension – to be on his side. In the paragraph preceding the one just quoted (Principles, §469), Ploucquet cites extensively from Malebranche’s Search after Truth (book IV, part. II, chapter 3; LO, 448–450). Next to the passage where Malebranche picks up on the matter of intra-mental causation – “They [the most noble minds] can determine the impression God gives them toward Himself toward objects other than Himself, I admit; but I do not know if that can be called power” (Search after Truth, LO, 449) – Ploucquet adds a Nota Bene on the margin (Principles, §469, 300). He refers to this Nota Bene in the following paragraph believing to be in agreement with Malebranche. This, however, seems to stretch Malebranche’s considered opinion. For Malebranche’s wholesale occasionalism including intra-mental causation, see for instance Schmaltz, Early Modern Cartesianisms, 209–227.

15 Elsewhere, I have comprehensively discussed the demise of occasionalism in eighteenth-century Germany. Both the increasing weight of naturalized and immanent rather than transcendental approaches to causation, and a shift from more metaphysical accounts of causation to causal explanation have eventually led to occasionalism’s end (Henkel, Grounding the World, chapter 4).