[This is part of a series of blog posts about articles in the new, open-access journal, Ergo]
Leibniz’s mill argument is one of very few Leibnizian arguments frequently invoked in contemporary philosophy of mind. How exactly this argument works, however, is controversial among Leibniz scholars. In the past few months, two stimulating articles devoted exclusively to the mill argument have come out: Marleen Rozemond’s “Mills Can’t Think: Leibniz’s Approach to the Mind-Body Problem” (Res Philosophica 91.1, 2014) and Paul Lodge’s “Leibniz’s Mill Argument Against Mechanical Materialism Revisited” (Ergo 2014). Rozemond’s paper was published first, but as Lodge acknowledges in a footnote, he only became aware of this paper after writing his own, and therefore does not otherwise engage with it. Hence, I’d like to put these two excellent analyses in conversation with each other here. In fact, even though the two papers disagree on several fundamental questions, they also turn out to help each other in interesting ways.
Let me start with the primary texts under discussion. The most famous formulation of the mill argument occurs in Monadology section 17:
we must confess that perception, and what depends on it, is inexplicable in terms of mechanical reasons, that is, through shapes and motions. If we imagine that there is a machine whose structure makes it think, sense, and have perceptions, we could conceive it enlarged, keeping the same proportions, so that we could enter into it, as one enters into a mill. Assuming that, when inspecting its interior, we will only find parts that push one another, and we will never find anything to explain a perception. And so, we should seek perception in the simple substance and not in the composite or in the machine. (transl. from AG 215)
Leibniz does, however, offer versions of this argument elsewhere as well, as both Rozemond and Lodge acknowledge. Particularly interesting are the versions from Leibniz’s Preface to the New Essays (NE 66f.), a letter to Bayle (G 3:68/WF 129), a draft of a letter to Queen Sophie Charlotte (LTS 259), and “On the Souls of Men and Beasts” (G 7:328/SLT 63). I will not quote those passages here, but they can be found in Rozemond’s and Lodge’s articles.
Turning now to the two recent discussions of the mill argument, I will start with Lodge’s because it provides a useful categorization of the different interpretations of the argument that have so far been advanced. The argument, Lodge claims, has the following structure:
Premise: Perception, sensation, and thought cannot be explained in mechanical terms.
Conclusion: Therefore, matter (as understood by mechanistic philosophers) cannot perceive, sense, or think.
Lodge then lists four different interpretations of the implicit justification for the argument’s premise. They can be paraphrased as follows:
- The Explanatory Gap Interpretation (Stewart Duncan): Shape, size, and motion are the only modifications of matter, and we cannot conceive how these modifications or their combinations could give rise to perception, sensation, or thought.
- The Unity of Consciousness Interpretation (Margaret Wilson): Conscious perceptions possess a special unity, and material things, since they are infinitely divisible, cannot exhibit, or give rise to, this kind of unity.
- The Unity of Perception Interpretation (Marc Bobro and Paul Lodge; Stewart Duncan): Perception can only take place in a unity, and material things, since they are infinitely divisible, cannot exhibit this kind of unity.
- The Activity/Passivity Interpretation (Marleen Rozemond; Paul Lodge): The power to perceive, sense, or think is an active power, and matter, since it is passive, cannot possess active powers.
It is important to note that the controversy over the mill argument is not primarily a controversy over what Leibniz’s views about perception or the possibility of thinking machines are. Interpreters in fact generally agree that Leibniz denies that machines are capable of thought or perception, and that he believes that only simple, immaterial unities could possibly possess perceptions and thoughts. Most scholars furthermore agree that because all natural states of a monad originate within the monad, perceiving involves some kind of activity. The controversy is, rather, about what exactly the structure of the various versions of the mill argument is. Even though this is not a disagreement about Leibniz’s fundamental views, it is an important interpretive issue, and not only because the mill argument is so frequently invoked in contemporary philosophy of mind. It is also important for evaluating how powerful and compelling Leibniz’s argument is, especially to readers who do not already accept large portions of Leibniz’s system. One crucial aspect of the controversy, then, is the question to what extent we already need to accept controversial Leibnizian doctrines in order to find the argument compelling. Relatedly, the controversy concerns the relations between Leibniz’s fundamental views, for instance between activity and perception. Even interpreters who agree that monads are active in perceiving, after all, may disagree on whether activity is a necessary condition for perceiving.
Lodge rejects the first of the four interpretations listed above as too minimalistic because he sees Leibniz as pointing to particular features of perception that make a mechanical explanation impossible. He also rules out the second interpretation, but on textual grounds: Leibniz seems to be concerned with perception generally, not conscious perception in particular. Yet, Lodge argues, the third interpretation is the best way to make sense of some versions of the mill argument, while the fourth interpretation works better for a few other versions.
I am not going to go into more detail of Lodge’s argument here. Instead, I will turn to Rozemond’s interpretation of the mill argument and end with some observations about the most significant differences between her reading and Lodge’s.
Rozemond argues that the activity/passivity interpretation is the best way to understand all versions of Leibniz’s mill argument, even the ones in the Monadology and the letter from Bayle, which Lodge thinks are better understood in terms of the unity of perception interpretation. She moreover adds a fifth candidate to the list of possible interpretations of the mill argument.
5. The Internal Action Interpretation (Marleen Rozemond): Perception is an internal action, which means that it cannot consist in the operation of various parts of an entity. Whatever a machine does, however, consists in the operation of its various parts, and therefore machines cannot perceive.
Rozemond provides convincing textual evidence that Leibniz uses ‘internal action’ in two different ways: sometimes it is contrasted with transeunt action, at other times it is contrasted with actions consisting in the operations of parts of the agent. She moreover suggests—plausibly, I think—that the latter understanding of the term ‘internal action’ is at work in passages in which Leibniz argues that matter cannot perceive because perception is an internal action.
This fifth interpretation appears to me to be closely related to the unity of perception interpretation. Determining just how closely they are related would require a much more thorough examination of how exactly Leibniz understands the unity of perception, and of what exactly he means when he calls perception an internal action. It may or may not turn out that they are versions of the same interpretation. Either result, however, would be interesting and advance our understanding of the mill argument.
If the internal action interpretation does not turn out to be a version of the unity of perception interpretation, Rozemond has discovered yet another plausible way of understanding the mill argument. This new interpretation might even solve some of the interpretive problems that the other candidates cannot handle convincingly.
If the internal action interpretation does turn out to be a version of the unity of perception interpretation, on the other hand, this very realization, and the examination that led to it, would presumably afford us a deeper understanding of what the relation between perception and simplicity or unity is for Leibniz. Moreover, we could then subsume at least some of the passages in which Leibniz invokes internal action and which Lodge subsumes under interpretation (4), under interpretation (3) instead. This would be interesting for Lodge, who understand some passages in accordance with the activity/passivity interpretation because they invoke the notion of internal action. On the basis of the textual evidence Rozemond presents that Leibniz sometimes uses ‘internal action’ to refer to an action not resulting from the operation of parts of the agent, one could argue that worries about unity or simplicity are after all doing most of the work in those versions of the argument. This strategy would work particularly well for the passage from a draft of a letter to Sophie Charlotte, which Lodge reads in accordance with the activity/passivity interpretation. It might also help explain why Leibniz brings up internal actions in the Monadology, directly after the mill argument, as well as in the letter to Bayle. This is one way in which Rozemond’s discussion helps Lodge’s argument.
Rozemond claims—correctly, I think—that in the texts she discusses, Leibniz does not explicitly identify what I call the internal action interpretation as underlying the mill argument. Instead, she argues that Leibniz sometimes brings up internal action as an additional reason for rejecting thinking matter, in addition, that is, to considerations about the activity of perception and the passivity of matter. Rozemond also wonders whether Leibniz might be relying implicitly on the internal action interpretation in some versions of the mill argument. Yet, she does not mention the draft of a letter to Sophie Charlotte, which Lodge discusses, and in which Leibniz provides a version of the mill argument that fits perfectly with the internal action interpretation. Leibniz there writes,
supposing whatever traces, machines, or motions you like in the brain, one will never find the source of perception or of the reflection on oneself, which is a truly internal action, any more than one could find it in a watch or in a mill. For crude or subtle machines differ only in degree. (Leibniz and the Two Sophies, p. 259)
This is one way in which Lodge’s discussion helps Rozemond: it supplies a version of the mill argument in which Leibniz explicitly employs the strategy Rozemond finds most promising.
There are many thought-provoking aspects of both Rozemond’s and Lodge’s paper that I was not able to explore here. For instance, Rozemond’s article includes an excellent discussion of the differences between Kant’s “Achilles Argument” and Leibniz’s mill argument; her paper also contains an argument against reading the mill argument in the Monadology in accordance with the unity of perception interpretation. I hence strongly recommend that those who are interested in the topic read both of these excellent papers and investigate these fascinating questions further. Even though the two articles have cleared up the main issues significantly, I agree with the last sentence of Rozemond’s paper: “Much work remains to be done.”