Leibniz’s Mill Argument Against Mechanical Materialism Revisited

3.1 Mechanical Explanation and the Explanatory Gap

Focusing on the mill passage from the Preface to the New Essays, Stewart Duncan argues that, in the Monadology and New Essays, Leibniz supports the premise by “trying to persuade [the reader] that . . . materialism is false using an intuition about inexplicability: ‘One cannot conceive . . . So . . .’” (2012: 258). The strategy that Duncan attributes to Leibniz is one that is familiar from recent discussions concerning the metaphysics of mind. Leibniz is represented as trying to get the reader to accept that consideration of the explanatory resources that mechanical materialism offers leaves one unable to see how there could be a way to develop an explanation of how such a system might perceive, sense, or think.[11] Importantly, Duncan’s Leibniz does not offer any positive characterization of the explanandum when making the claim about inconceivability. The only contribution that the terms perception, sensation, and thought make to the argument is to indicate phenomena to which readers should attend so that they will acknowledge the existence of an unclosable explanatory gap and agree that those phenomena are inexplicable in mechanical terms.

One obvious concern about attributing this explanatory-gap interpretation to Leibniz is that it leaves him open to the charge that the inexplicability of perception, sensation, and thought may be merely apparent. Duncan is sensitive to this, but presents evidence that Leibniz regarded some instances of inconceivability as providing grounds for claims about what is possible, and hence about what is actually the case. In light of this, he points out that whilst Leibniz says that “we must not deny what we do not understand,” he also claims that “we are entitled to deny (within the natural order at least) whatever is absolutely unintelligible and inexplicable” (NE: 65). Duncan also draws attention to the fact that Leibniz says a little more about what would be required for there to be an intelligible account of the relationship between a feature, or modification, of something and that thing, namely that the modification “must arise from limitations and variations of a real genus, i.e. of a constant and absolute inherent nature” (NE: 65). Thus, Duncan’s reading of the argument of the Preface can be recast in the following way: The modifications of mechanical matter are exhausted by the size, shape, and motion, and their mechanical combinations. Conceiving of the nature of these modifications and their combinations (aided by the mill) leaves us unable to see how they might provide the basis for an explanation of perception, sensation, and thought. From this, we can infer that perception, sensation, and thought are inexplicable in mechanical terms and, ultimately, that they are not modifications of mechanical matter.

Despite these claims, Duncan still recognizes a prima facie problem with interpreting Leibniz in this way. He notes that, whilst Leibniz provides some positive instances of intelligible modifications, such as the shape of a body being a limitation of its nature, i.e., extension, he (Leibniz) does not provide general criteria for determining when something is of this kind. Thus, Leibniz still leaves himself open to a version of the original worry that any given claim of unintelligibility may be an instance of something “we do not understand”, rather than something “absolutely unintelligible and inexplicable.” For even if we accept that Leibniz is right in claiming that we cannot conceive how mechanical features might combine in order to give rise to perception, sensation, and thought, why should we infer that it is genuinely inconceivable, i.e., that there could never be a way of conceiving of the relationship between mechanical matter and perception, sensation, and thought that was explanatory?[12]

These considerations notwithstanding, Duncan still regards his interpretation as the best reading of the mill argument in the Preface to the New Essays and Section 17 of the Monadology. The main reason for this is that Leibniz says very little in these passages beyond drawing attention to the inexplicability of certain aspects of mentality.[13] As we have seen, in the latter, Leibniz begins with the claim that “we must confess that perception, and what depends upon it, is inexplicable in terms of mechanical reasons” (GP: VI, 609/AG: 215) and then moves on to the mill. But, although the mill provides a vivid example of a mechanical system at work, its introduction is not accompanied by an explanation of why a machine could not perceive or have the other features that are said to depend on perception. And the passage from the Preface to the New Essays simply runs as follows:

Duncan draws attention to an important feature of almost all of the places in which Leibniz discusses the mill, namely, that he is frustratingly vague about what his reasoning might be. And this clearly invites the thought that it should be understood in the minimal way that Duncan suggests. However, there is textual evidence that we should look for something more inflated. Of most significance here is Leibniz’s Reply to Bayle, written in 1702 (i.e., around the same time as the letter to Bayle that includes the mill), in which he expresses the view that arguments based on inexplicability require a positive ground. Thus, he writes: “It is no proof of the impossibility of something merely to say that one cannot conceive this or that, when one doesn’t make clear where it conflicts with reason, and when the difficulty is only one of imagination, and not of understanding” (GP: IV, 565/ WF: 119). There is reason then to look for ways in which Leibniz might support the premise of the mill argument by articulating his conceptions of the explananda in such a way that claiming that they are explicable in mechanical terms “conflicts with reason.”

The three remaining interpretations that I wish to consider all have this desired form. I shall argue below that two of the three have some plausibility. But there are important caveats. I think there is evidence that Leibniz explicitly employed the third of these lines of reasoning in some of the passages in which he presents the mill and argues against mechanical materialism. However, this interpretation does not seem to account for what is going on in the Monadology, and its presence in the Preface to the New Essays is certainly open to question. Furthermore, whilst I still wish to suggest that Bobro and I were right to think that the second of the remaining interpretations provides a plausible way of thinking about Section 17 of the Monadology and the letter to Bayle from 1702, I now think that the case for this reading was overstated in our previous discussion.

3.2 Mechanical Explanation and the Unity of Perceptual Consciousness

The first interpretation that I want to discuss is due to Margaret Wilson, who offers it as part of an analysis that is intended to provide an interpretation of Section 17 of the Monadology. She writes:

Wilson’s suggestion is that when Leibniz talks of the inexplicability of perception in the Monadology, he is really concerned with the inexplicability of “the unity of perceptual consciousness”. She elaborates the idea as follows:

Although Wilson’s understanding of the expression “‘unity’ of consciousness” is crucial here, she does not provide any further elaboration when she introduces her reading. However, considerations toward the end of her article suggest she is thinking of a kind of unity that is intrinsic to conscious sensory perception, qua conscious.[15] For here she claims that contemporary materialists who locate the experiences associated with each of the sensory modalities in different parts of the brain will face a problem similar to the one raised by Leibniz in that they will “not have explicated the fact that seeing-green-and-smelling-cabbage is one experience with distinguishable components, or in other words, that the sensations belong to one consciousness” (1999: 400).[16]

As Bobro and I argued, this reading does not seem plausible.[17] An initial problem is that, at least as far as I am aware, there is no positive textual evidence that Leibniz is concerned with the kind of unity mentioned by Wilson. Moreover, what is said to be inexplicable in Section 17 of the Monadology, i.e., perception, is explicated in a way that directly undermines the reading three sections earlier in Section 14 of the same work. Here Leibniz observes that “[t]he passing state which involves and represents a multitude in the unity . . . is nothing other than what one calls perception” (GP: VI, 608/AG: 214).[18] Implicit in this observation is something that is made explicit in The Principles of Nature and Grace, which is contemporary with the Monadology, namely that conscious awareness is not essential for perception as Leibniz understands it. Leibniz writes in the Principles:

Two things are clear here: First, Leibniz does not regard all perception as involving apperception, or as self-conscious perception; and second, Leibniz claims that there are souls that perceive without apperceiving.[20] Taken together these considerations strongly suggest that consideration of a unity that is intrinsic to conscious perceptual awareness, qua conscious, is unlikely to be that which leads Leibniz to regard perception as inexplicable in mechanical terms.

3.3 Mechanical Explanation and the Unity of Perception

Whilst I have argued that there are problems with Wilson’s focus on the unity of perceptual consciousness, it may already be apparent that her discussion draws attention to aspects of Leibniz’s conception of perception that provide the basis for another reading of Section 17 of the Monadology. As we saw, in Section 14 of the Monadology, Leibniz characterizes perception as “[t]he passing state which involves and represents a multitude in the unity” (GP: VI, 608/AG: 214). Furthermore, machines are systems comprised of matter that is essentially extended, and, therefore, essentially divisible. Given this, the following grounds for the claim that there could never be a mechanical explanation of perception emerge: Since mechanical explanations always advert solely to properties of an essentially divisible mechanical matter, machines and their properties could never be characterized in such a way that it could be said that the machine is a unity. Thus, no mechanical explanation could capture an essential component of what is required for perception to occur.[21]

This reading is not without its problems, given that Leibniz does not mention unity at all in Section 17. But it still seems to me reasonable to think that it may have been what Leibniz was thinking. For one thing, only three sections earlier we find unity offered as an essential feature of perception; for another, toward the end of Section 17, Leibniz draws attention to the fact that the machine in which “one should not seek perception” is “composite” (GP: VI, 609/AG: 215). Also, it should not be forgotten that the Monadology opens, in Section 1, with a definition of the monad as “a simple substance . . . – simple, that is, without parts” (GP: VI, 607/AG: 213). Thus, the resources are readily available in the early parts of the Monadology for the reader to understand Section 17 in this way. And, finally, Leibniz offers no explicit alternative. [22]

One might at this point fall back on Duncan’s attribution of the inexplicability reading, albeit on pain of ignoring Leibniz’s claim in the Reply to Bayle that we need positive reasons to rule out a possibility on the grounds of its being inconceivable by us. Or perhaps one might wish to suggest that we simply do not know what positive reason Leibniz had in mind in Monadology Section 17. Both of these options seem compatible with the text of the Monadology. Nonetheless, my own inclination is still to think that the unity of perception may well be doing the work.

Further evidence that Leibniz saw the connection between consideration of the mill, the unity of perception, and the rejection of mechanical materialism is offered by Duncan. The evidence in question is found in his letter to Bayle of 1702. In the context of a report of a conversation that Leibniz had had with John Toland about Toland’s “claim that matter can become able to think, as it can become round, and thus that a certain organization, or a certain shape, can produce thought, and that when that organization is destroyed, thought will cease” (GP: III, 68/WF: 129), Leibniz tells Bayle:

And later in the same paragraph Leibniz presents his definition of perception as “the expression of a multitude in a unity” (GP: III, 69/WF: 130),[23] making it clear that this is the notion of perception that he has in mind. As Duncan notes, there is nothing so far that provides any real purchase on why it is that Leibniz thinks that we would not find “the origin of perception” in a machine or “never [understand] that it might perceive” (2012: 254). However, the passage continues in ways that suggest to Duncan, and to me, that the unity of perception may have been on Leibniz’s mind. Thus, we find:

Here Leibniz suggests that mirrors (and perhaps “brain traces”) have the capacity to produce accurate images of things, and, in so doing, get as close to thought as anything visible could. But they are still said to be “infinitely far away from” and “absolutely heterogeneous” with it.

As Duncan argues, Leibniz is happy to regard images such as those we might find in a mirror as expressions (or, as Leibniz sometimes puts it, “representations”) of other things.[24] But the crucial difference between other forms of expression and perception is that the latter occur in simple things or “unities”, a fact which, as we have seen, Leibniz goes on to highlight shortly after. It is true that Leibniz does not explicitly say that this grounds the heterogeneity between mirror images and brain traces on the one hand and perception and thought on the other. But I do not find it too much of a stretch to think that this was the “conflict with reason” that underwrote Leibniz’s belief that we could never understand mentality in mechanical terms here.

Nonetheless, in his recent paper, Duncan has raised a challenge to Bobro’s and my earlier suggestion that this line of reasoning is present in Section 17 of the Monadology.[25] This stems from his understanding of the role that Section 17 is playing, namely as the final step in an argument that is supposed to establish the claim that perception is the representation of many in a unity. Clearly if this is right, it would provide compelling reasons not to think of the argument of Section 17 as relying on this account in the way that my reading does. But I am not sure that we should regard it in the way that Duncan suggests.

If we are to agree with Duncan’s interpretation, we need to interpret Sections 14–17 of the Monadology as follows: Leibniz introduces a definition of perception as the representation of a multitude in a simple substance in Section 14. He then uses Section 16 to persuade those who accept that the soul is simple also to accept his account of perception, on the grounds that they are aware of this in themselves. Finally, he uses an argument that appeals to the readers’ recognition of an explanatory gap to rule out the only other live option for those who do not accept that the soul is simple, namely those who advocate mechanical materialism.

But it is not clear that Section 14 is really functioning in the way that Duncan requires. Back in Section 2 of the Monadology, Leibniz establishes the existence of simple substances. From then on, he argues for a number of claims about the nature of these substances, and by the end of Section 12 he has argued that simple substances are the source of their own changes and must contain some kind of “diversity”. Section 13 then runs as follow:

It is immediately after this, at the beginning of Section 14, that Leibniz observes: “The passing state which involves and represents a multitude in the unity or in the simple substance is nothing other than what one calls perception” (ibid.). This is not so much a newly introduced definition of perception as an identification of perception with something that has already been presented as an essential feature of simple substances. With this in mind, we can turn to Section 16:

Here the claim is that those who agree that the soul is a simple substance should be happy to acknowledge that there are things that fit Leibniz’s description of simple substances more generally, or monads. After all, they should recognize that they themselves are such things.

But what of those who are tempted by mechanical materialism? It seems to me that whilst they are, as Duncan claims, the target of Section 17, they are not being persuaded to accept a definition of perception. Rather, they are being asked to acknowledge, on the basis of the initial claim from Section 16, that the perception they experience in themselves is something that is occurring within a simple substance rather than a machine, and, as result, again to accept that monads are as Leibniz has described them. Why would they do this? Because they apperceive, or have “reflective knowledge” of particular states that have multiple objects and which they thereby know must be unified in a way that could never be true of something that arose mechanically.

There is much here that might sound familiar from the discussion of Wilson’s interpretation above. But I am not evading Duncan’s challenge only to collapse the distinction between her reading and the one I favour. There is a crucial difference. What I am suggesting is that this passage appeals to the conscious experience of the reader as a bearer of states that unify a manifold, rather than an experience of themselves as having conscious states that have a unity, qua conscious states.[26] This reading requires us to hear the passage with an ear that is perhaps quite foreign in light of Kant’s famous separation of the notion of the unity of apperception from the notion of an entity that is a genuinely unified subject of representations. For it requires that we allow Leibniz to assume that his readers will acknowledge an awareness of themselves as perceivers, the object of which is distinct from the conscious awareness that they are. But we should not confuse the doubts that Kant’s distinction might cast on the reasonableness of Leibniz’s claims with the thought that this would have been a concern to Leibniz or his contemporaries. It seems to me, then, that the interpretation of the mill argument of Section 17 of the Monadology as based on considerations of the unity of perception can be defended against Duncan’s worry.

As I acknowledged above, this reading of the Monadology and of the letter to Bayle must be treated with some caution. In neither case does Leibniz explicitly mention unity in the places where he presents the mill. But the surrounding text seems suggestive enough to me in each case. And, even if Leibniz did not put the consideration to quite the use that I am claiming, at the very least the unity of perception is a reason that Leibniz could have offered for the claim that nothing material could perceive, sense, or think.

3.4 Mechanical Explanation and Activity

For all that I have argued that the occurrences of the mill argument in the Monadology and the letter to Bayle are to be read in the way outlined in the previous section, I do not think that this exhausts the ways in which the argument should be understood. This is something that is borne out by considering Leibniz’s discussion of thinking matter in a number of passages that, as far as I am aware, do not appear in any other treatment of the issues. One of these passages appears in the main body of the New Essays, in Book 4, Chapter 6, where Leibniz returns to the topic of thinking matter.

Leibniz’s reasons for rejecting the natural activity of matter in Book 4 emerge as an explicit response to Locke’s claim that matter might think where “Omnipotency has . . . given to some systems of matter fitly disposed, a power to perceive and think” (ECHU: 4.3.6/NE: 378). In considering this possibility, Leibniz observes: “I believe you agree, sir, that it is not within the power of a bare machine to give rise to perception, sensation, reason” (NE: 379). Given the dependency between perception, sensation, and thought that Leibniz articulates in the Monadology, one option would be to suggest that it is Leibniz’s understanding of perception as involving unity that is again driving his rejection of mechanical materialism. However, closer examination of what follows reveals another possibility.

In book 4, following Locke, Leibniz couches his argument in terms of powers. In considering whether matter might acquire the power to perceive, sense, and think, Leibniz takes himself to be in agreement with Locke that a “bare machine”, of which both the mill and the watch from the Preface are examples, cannot have such a power. Before pursuing what Leibniz says at this point, it is worth pausing to consider what he says about the notion of power elsewhere in the New Essays. At the beginning of the chapter “‘Of power’ and freedom” (NE: 168–170), after presenting a very condensed version of Locke’s account of power from ECHU (2.21.1–2), Leibniz responds:

Here we see that the powers of a thing are its “possibilities of change” and that these come in two kinds, a thing’s active powers (which are also called faculties) to give rise to change and the passive powers (capacities or receptivities) to receive change.

These distinctions are pertinent when we consider the text immediately preceding Leibniz’s observations about his and Locke’s agreement in Book 4, Chapter 3, Section 6. Here we find:

The power to think is a faculty or active power. And here Leibniz observes that faculties are “ways of being” (a notion that is equated with “modification” a few sentences later) that are not derivable from matter considered in the Lockean way, i.e., mechanical matter. Furthermore, his point appears to be that it is the fact that such matter is passive that prevents this. As we have seen above, modifications are merely limiting variations of some nature or other for Leibniz. The problem that he identifies here seems to be that something made of matter could only perceive, sense, or think naturally if it could come to have the power to perceive, sense or think, i.e., if it could come to have a modification that was an active power.[27] This would require that something entirely passive become active through mere limitation. In other words, it would require precisely the augmentation that Leibniz rules out in a contemporary letter to De Volder (dating from 19 November 1703) when he observes that modifications “only limit things and do not increase them, and therefore they cannot contain an absolute perfection that is not in the thing to be modified” (LDV: 277).

Perhaps a little surprisingly, the claims made here about the power “to give rise to perception, sensation, reason”, appear to be parasitic on a more general one. Since mechanical matter is purely passive, any modifications (including any “faculty”) that might belong to something made of mechanical matter would be passive too. Thus, whilst a machine might have what Leibniz calls a “‘capacity’ or ‘receptivity’” (NE: 169) for motion, it could not have the kind of power required to give rise to any kind of change, including the coming into being of cognitive states.

The suggestion that this is the consideration that Leibniz relies on here is given further support in two ways. Firstly, it also allows us to see why Leibniz appears so confident that Locke and he agree on this issue. For, in the correspondence with Stillingfleet, Locke rejects the idea that matter “as matter” can come to have the power of thinking on the grounds that it is “devoid of all activity” (NE: 65), as Leibniz reports. Secondly, and more importantly, this consideration is made explicit in other passages in which the mill occurs, which have not been discussed in the secondary literature to this point, however.

The first such passage is in a draft letter to Queen Sophie Charlotte dating from 1702:

Whilst the passage is somewhat unclear, one might think that perception and self-reflection (a necessary condition for thought) are being ruled out due to the facts that they involve internal action and that matter is essentially passive. A second passage from “On the Souls of Men and Beasts” is perhaps more explicit. Leibniz begins the piece by noting that “matter taken in itself, i.e., bare matter, is constituted from antitypy and extension” (GP: VII, 328/SLT: 63). In the next paragraph, he adds that “it is obvious that matter is something merely passive, since its attributes and variations involve no action” (ibid.). Next comes the mill:

This passage could not be much clearer in its endorsement of the interpretation that I have offered in this section.[29] It seems plausible to think, then, that we have found in the New Essays and these additional texts another reason why Leibniz thinks that mechanical matter cannot have perception, sensation, and thought. Even the most complex machine would merely be a system of passive matter whose changes were motions initiated from without or through a redistribution of such motions. In no sense could such a machine have or acquire the capacity for genuine activity, as would have to be the case if it could naturally have or acquire a faculty of perception, sensation, or thinking.

Once alerted to the existence of this interpretation, one might wonder whether it is also at work in some of the other passages that we have already considered. Talk of action can indeed be found in the Monadology, with Leibniz observing at the end of Section 17 that it is in “perceptions and their changes . . . alone that all the internal actions of simple substances can consist” (GP: VI, 609/AG: 215). However, the fact that perception involves activity is an afterthought rather than part of the argument of that section. The letter to Bayle is somewhat different. For here Leibniz observes:

But although Leibniz seems to be endorsing the activity of thought as a basis for rejecting mechanical materialism, this passage occurs immediately after he has discussed the heterogeneity of machines and perceivers. Thus, whilst it provides additional support for the reading I am offering in this section, Leibniz seems to be suggesting that this consideration is distinct from whatever he had in mind in the discussion following the introduction of the mill in the letter to Bayle.

The case of the Preface to the New Essays is more complicated. The passage is very difficult to interpret. As we have noted, it is like Section 17 of the Monadology in that it contains no explanation of why it is that sensation and thought (there is no mention of perception in this case) are inexplicable in mechanical terms. Assuming we reject the explanatory-gap reading on the grounds given above, we again have the option simply to remain agnostic about Leibniz’s reasons. And it may be that this is the safest strategy. However, it is worth at least entertaining the thought that this may be one of the cases in which Leibniz has the activity of sensation and thought in mind.

I think that two reasons speak in favour of this. The first is the fact that this passage is explicitly addressing the issue of thinking matter. As we have seen, when Leibniz returns to discuss this topic in the body of the text, it is the passivity of mechanical matter and activity of perception, sensation, and thought that drive the argument. The second is the fact that the mill passage appears in close proximity to a passage that presents an analogous attack on Newton’s conception of gravity, which runs as follows:

Here we again find Leibniz concerned with the fact that an active power is being ascribed to a mechanical system, namely the power to attract bodies. Whilst it is true that he does not explicitly mention the passivity of matter as the source of inexplicability, Leibniz’s initial introduction of the issue is prefaced by an explicit reference to Locke’s admission in his correspondence with Stillingfleet that he conceives of natural operations of matter as limited to impulse alone.[30] These considerations are not absolutely decisive, but they do provide us with a way of making his claims intelligible, given that we have seen that Leibniz explicitly regards the passivity of mechanical matter as inconsistent with activity in other places.