Leibniz, Berkeley, Kant, Frege; bees, toasters and Julius Caesar

Jeffrey K. McDonough interviewed by Richard Marshall.

Jeffrey K. McDonoughis the Laphroaig single malt of Leibniz and Berkeley studies, bringing them all back home over a peaty fire. He's forever brooding about Leibniz's theodicy, his systematicity, his monads, his views on monadic causation and the relation betwen the divine and creaturely activity, how Leibniz thought God and physics were compatible, about teleology and the mechanistic universe, about the role of optics in his philosophy, about Berkeley's idealism, about what a bee is to Berkeley, about concurrentism, about its difference from Malebranche's occasionalism, about Kant's refutation of Descartes, and why working out why Julius Caesar is not a number is philosophically useful. Best savoured neat, or with a little cool water...

3:AM:What made you become a philosopher?

Jeffrey K. McDonough:Oh, I don’t think I had much choice in the matter. Like most American students, I didn’t take any philosophy courses in grade school or high school, but I did have a couple of classes in which we read “great books.” In one junior high class, we read bits of the Nicomachean Ethics, and I remember being eager to tell my parents about this amazing Aristotle guy. In my favorite class in high school – explicitly a great books course – we read the Apology as well as some pretty philosophical pieces of literature, and just talked and wrote about them. It was great! That was enough to get me into an introductory philosophy course my freshman year in college. I still had wide interests and took lots of different courses, but somehow I managed to find room for a philosophy course every quarter and by the time I was a junior it was pretty obvious that I was hooked.

3:AM:You’re a go-to guy on Leibniz. I guess we ought to plunge right in and ask about his theodicy. Can you sketch out its most salient features first of all and tell us whether he’s a systematic philosopher or not?

JM:There are a lot of details and subtleties in Leibniz’stheodicy, but its most salient feature is the thesis that this is the best of all possible worlds. Leibniz’s core thought is that if this is the best of all possible worlds, then – assuming that the world is good at all! – God can’t be blamed for creating it. Indeed, if this is the best of all possible worlds, God might seem blameworthy if he didn’t create it.

Now, of course, it’s not obvious from just looking around that this is the best of all possible worlds. Apparent tragedies and injustices abound, as Leibniz well knew. He grew up in the shadow of the miserably destructive Thirty Years War and worked throughout his life to promote peace among the rival Christian sects of his day.

Leibniz’s theodicy, however, isn’t driven by everyday experience but rather by faith and reason. He maintained that what seem like imperfections in the world are really potentially misleading pieces of a larger puzzle that it is hard, perhaps impossible, for us to grasp as a whole. I don’t myself find Leibniz very convincing on this point, but we shouldn’t be dismissive of his rationalism. While I think atrocities are real and speak – perhaps decisively – against the perfection of the world, one doesn’t discover, say, the infinitesimal calculus or universal laws of nature by relying on everyday experience alone.

Leibniz was one of the last – perhaps the last – great polymaths. He made significant contributions to an almost unbelievable range of fields, including physics, mathematics, law, politics, theology, literature, history, linguistics, and anthropology. He even wrote poetry! Furthermore, he enjoyed an unusually long career, starting young and living to be almost seventy. So it’s natural to wonder if Leibniz’s thinking is consistent not only with respect to particular projects – such as his theodicy – but also across different domains and through time.

I think it’s pretty clear that Leibniz thought of himself as a systematic thinker. In correspondence, he often refers to his “system” and – not always with great modesty – trumpets connections between his far-flung studies. Not everyone has been convinced however. Some prominent recent scholars such as Catherine Wilson, Daniel Garber, and Glenn Hartz, for example, have suggested that there are deep, irresolvable tensions in Leibniz’s thinking.

For what it’s worth, I’m an optimist with respect to Leibniz’s claims to systematicity. In spite of his tendency to express his views in different ways, especially to different audiences, he was a remarkably systematic thinker, and I’m still find myself regularly surprised by how different aspects of his thought turn out to be related to one another in interesting, non-obvious ways.

3:AM:What does Leibniz think is the fundamental metaphysical ontology – I guess this comes down to asking what account of substance does he have?

JK:In his best-known, most mature works, Leibniz argues that the most fundamental things in the world – the world’s “substances” – are immaterial simples or “monads.” Monads are essentially modeled on human minds or souls, although Leibniz maintains that some monads must be more perfect than our souls and others must be less perfect. He maintains that each monad is metaphysically simple, indestructible, and represents the whole of the world from its own distinctive point of view.

There is, however, another account of substance to be found in Leibniz’s writings. What he calls “corporeal substances” seem to be modeled on living creatures – worms, fish, embodied human beings. Corporeal substances have some properties in common with monads or “incorporeal substances.” Leibniz maintains, for example, that organisms don’t really die. He suggests in apparent death, they shrink dramatically and enter dormitive states, something like spores awaiting a new opportunity to spring back to “life.” He thus maintains that there is a sense in which corporeal substances, like incorporeal substances, are naturally indestructible.

Now, of course, we might well want to know how Leibniz’s two accounts of substance are supposed to fit together. The standard view among commentators has been that monads represent the deepest foundations of Leibniz’s metaphysics. On this account, corporeal substances are held to be non-foundational entities. While corporeal substances have some properties in common with the world’s true substances, they are at the end of the day second-rate, derivative beings.

An alternative account, developed most prominently by Daniel Garber, suggests that for much of his career Leibniz takes very seriously the thought that the world is ultimately grounded not in incorporeal mind-like entities, but rather in extended, organic entities. On this interpretation, corporeal substances aren’t second-rate, derivative entities, but rather first-rate, alternatives to incorporeal substances.

My own view falls somewhat in between these two interpretations. Leibniz thinks his philosophical system is consistent with both corporeal and incorporeal views of substances. He thinks that inorganic bodies are made up of organic bodies, and that organic bodies are made up of incorporeal minds or souls. This is especially important to him because both accounts of substance are associated with competing theological visions of human nature. The corporeal substance account is associated with a picture of human beings as essentially unions of minds and bodies; the incorporeal substance account with a picture of human beings as essentially immaterial souls.

3:AM:So what are his views on monadic causation and the relationship between divine and creaturely activity?

JM:Leibnizthinks that the world can be exhaustively analyzed into incorporeal, mind-like monads. For various metaphysical reasons, however, he thinks that monads can’t causally interact with one another. Each monad causally unfolds autonomously. It nonetheless seems like things causally interact with one another because the causal unfolding of each monad is perfectly synchronized by God with the causal unfolding of every other monad. I will my hand to go up and it goes up. But my willing doesn’t cause my hand to go up. The two events are causally independent, but, thanks to the pre-established harmony, they happen one after the other just as they would if they were causally related.

Leibniz’s account of monadic causation is further complicated by once standard views concerning the relationship between divine and creaturely activity. In earlier times, philosophers insisted that creaturely activity depends upon divine activity in at least three distinguishable ways. First, God has to create creatures that can act. Second, he has to sustain or “conserve” those creatures so they don’t cease to exist. Third, God has to actively assist creatures in the production of their effects; without God’s “concurrence,” a match couldn’t light a candle, and a candle couldn’t illuminate a dark room.

It is pretty clear, I think, that Leibniz accepts all three traditional doctrines of creation, conservation and concurrence. What has been less clear is how Leibniz’s commitment to divine conservation and concurrence are supposed to intersect with his views on the activity of monads. Sukjae Lee, for example, has argued that Leibniz’s commitment to divine conservation and concurrence shows that Leibniz doesn’t think that monads are genuine efficient causes at all. I’m a fan of Sukjae’s work but I disagree with him on this point. I think that Leibniz believes – and is right to believe – that his commitment to the doctrines of conservation and concurrence are perfectly consistent with his commitment to monads being efficient causes.

3:AM:How does this God stuff intersect with his philosophy of physics? Isn’t piety fatal for any idea of a new science?

JM:Leibniz was both an important physicist and a committed theist. By his own account, he set himself to developing the new mechanical philosophy sometime around the age sixteen. Within ten years he had produced a strikingly original, if slightly crazy, physics of his own. He then traveled to Paris and received a crash course in the cutting edge physics and mathematics of his day. Shortly afterwards, he began making really big contributions in math and physics, some of which I suspect still aren’t fully appreciated today. Among other things, he discovered the infinitesimal calculus, offered a devastating critique of Descartes’s laws of motion, and laid the foundations for important developments in the eighteenth century by the likes of Euler, Lagrange and Jacobi.

Leibniz’s commitment to theism, however, runs even deeper than his commitment to the new science. A number of prominent scholars, including Donald Rutherfordand Maria Rosa Antognazza, have argued that essentially all of Leibniz’s various intellectual projects are ultimately driven by his overarching goal of articulating the glory of God and his creation. As remarkable as that may sound, I think they’re basically right. Bertrand Russell once spitefully suggested that Leibniz really has two philosophies, one that is confused and panders to religious authorities, and one that is clear-headed but inconsistent with traditional religion. But that’s nonsense. No one today, I think, could reasonably doubt that Leibniz was a theist who took religion very seriously; indeed, I suspect he took it more seriously than anything else.

As to how Leibniz’s interests in physics and theism fit together, I imagine that he would have been surprised by the common assumption today that physics and theism should be in tension with one another. Almost all of the great scientists in Leibniz’s day were theists. The great chemist Robert Boyle, for example, was very much involved in having the Bible translated into vernacular languages and endowed a lecture series – which still continues today – for the defense of Christianity against the threat of atheism. Likewise Isaac Newton, although he held some fairly unorthodox beliefs, wrote far more on religion than on science. At any rate, Leibniz saw science and religion as being complementary. In his view, science should aid our appreciation of God’s nature and existence while faith should spur us to inquire further into the marvels of the natural world.

3:AM:Teleology is a feature of his philosophy isn’t it – but surely this makes any idea of a purely mechanistic universe doomed. Isn’t Leibniz forced to have a ‘two-worlds’ position because of this? Or are Leibniz’s views on teleology subtler than modern commentators assume – just as some philosophers are now saying Descartes’ views on teleology are also – so we don’t have to have the two worlds view?

JM:I think teleology is a huge feature of Leibniz’s philosophy and that we can see it playing an especially important role in three distinguishable domains. First in the divine domain: Leibniz’s god is a providential god – he doesn’t just efficiently cause the world to exist, but chooses to create the actual world because it is the best of all possible worlds. Second in the domain of created substances: substances for Leibniz are modeled on living things, and all living things act not merely as efficient causes but also for the sake of ends; birds, bees and beekeepers don’t just bring about changes in the world, they bring about changes for the sake of other things. Third – and most controversially – teleology plays an especially important role in the domain of nature itself: Leibniz maintains, for example, that we have eyes for the sake of seeing and that there are laws of nature that bring about effects because those effects are optimal.

Now it’s a good question, of course, how Leibniz’s commitment to teleology is supposed to be reconciled with his commitment to an efficiently driven, broadly mechanistic universe. One might suppose that Leibniz means to posit two distinct “worlds,” most plausibly a world of minds that act teleologically and a world of bodies that act efficiently. While there are some texts that might encourage a two-world view, I don’t think that Leibniz actually holds such a position. As you suggest, Leibniz’s views on teleology are subtler than the two worlds picture would have it.

Consider divine teleology again. No doubt, Leibniz thinks that God is both the first efficient and the first final cause of the world’s existence. But there’s no question here of dividing divine agency into two realms – it’s not like there is one realm of divine efficient causation and another realm of divine teleological causation. So, at least with respect to God, Leibniz must think that efficient and final causes are compatible.

But that ought to encourage us to think more carefully about the actions of created substances as well. While Leibniz sometimes emphasizes that created substances also act for the sake of ends, I don’t think that he ever means to deny that created substances are also efficient causes. Created substances like you and I bring about effects not only efficiently but also for the sake of ends. To borrow from a passage in the Phaedothat Leibniz greatly admired, I wouldn’t be in a coffee shop now if my legs hadn’t moved efficiently in certain ways, but I also wouldn’t be here now if I hadn’t thought that I could get my morning caffeine fix here.

That still leaves the question of how teleology might relate to ordinary bodies. Suppose a ball rolls down a hill. It’s easy to imagine efficient causes of the ball’s movement, say, its being hit by another ball or the gravitational force exerted by the Earth. Might the ball’s movement also have a final cause? Well, the ball might have a derivative final cause. Perhaps a child has rolled the ball down the hill in order to return it to a friend. But Leibniz’s work implies that we might also think of the ball as rolling down the hill in order to minimize its potential energy. If that’s right, it might be possible to think of every physical event or action as being the result of both efficient and final causes. Rather than two distinct realms, we would have one world that may be viewed either from the vantage of efficient or final causation.

3:AM:Optics seems to have been a big deal for thinkers of Leibniz’s time. How come? And was it important for Leibniz’s philosophy or a side issue?

JM:I think optics was a big deal in Leibniz’s time for lots of different reasons. People have always been fascinated by light. It plays an important role in theological and philosophical analogies. It is bound up with vision and perception. Most importantly for my interests, however, optics – the study of light and its behavior – made remarkable progress in the early modern era. Galileo, Kepler, Descartes, Newton and Huygens all made important contributions to its development in Leibniz’s day.

Leibniz clearly wanted in on the action. His engagement with optics spans his long career – from the time of his earliest physics, circa 1671, through at least his mature Tentamen Anagogicumin the late 1690s. His biggest contribution to the development of optics is probably to be found in his defense of Fermat’s general approach to deriving the law of refraction. Fermat had proposed – in contrast to Descartes’s mechanistic approach – to derive the law of refraction from the principle that light takes the quickest path between its source and end point. While taking into account objections from contemporary Cartesians, Leibniz defended the spirit of Fermat’s proposal and helped to show how Fermat’s results could be arrived at more easily by making use of the infinitesimal calculus.

Leibniz’sinterest in optics, however, also had a large and formative influence on his thinking more generally. Leibniz’s mature physics emerges sometime around the late 1670s to early 1680s, and when it does it draws in crucial ways upon his efforts to derive the laws of optics. Furthermore, his work in optics provides him with a precise and structured way of thinking about optimality that carries over into his philosophical thinking about divine, creaturely, and even natural teleology. Leibniz’s engagement with optics thus turns out, I think, to be central to the development of his scientific and philosophical thought.

3:AM:Given that he was a genius and most of the people involved in discussing the issue of the vexed relationship of physics and religion, and/or metaphysical and naturalist philosophy, aren’t, are any of his arguments for the reconciliation of religion and scienceany good for our contemporary situation?

JM:That’s a tough question. As I said, I don’t think that Leibniz saw a tension between science and religion. Indeed, just the opposite. He thought that science helps to reveal just how marvelous creation really is and in doing so helps support theism against atheism. While I don’t think that he has any specific arguments for reconciling religion and science that are still applicable today, I do think that his own life and efforts might suggest three lessons for people today.

First, as a great scientist and devout theist, Leibniz might serve as a reminder that rational, scientific minded thinkers have long thought that religion and science are compatible. Theists might be wrong in their beliefs, but I think it would be foolish to dismiss them as simply irrational or superstitious.

Second, in spite of being a devout theist, Leibniz was consistently open-minded towards other religions and even atheists. He found much to admire, for example, in Jesuit reports of religious beliefs in Chinaand clearly had a profound respect for the philosophical thought of the most notorious atheist of his day, namely, Baruch Spinoza.

Third, scientific discoveries in Leibniz’s time had begun to undermine the view that human beings occupy a unique, vaunted place in the universe. Rather than attack those scientific discoveries, Leibniz embraced them, concluding that the truth of theism doesn’t mean that it’s all about us. That strikes me as a healthy thought and one that is likely to be especially important for addressing the challenges of the century to come.

3:AM:Leibniz isn’t the only guy you’ve written about however. You’ve brooded on Berkeley and what he thought ordinary objects like tables and bees are, given that he thought reality was fundamentally just spirits and ideas. So what did he think a bee is?

JM:As you say, Berkeley was an idealist: he thought that fundamental reality – apart from God – is exhausted by finite minds and their ideas. So what is a bee? The physical bee that I see and hear obviously can’t be a mind. It’s tempting to suppose that it must therefore be a particular idea. But ideas are private for Berkeley. If bees were particular ideas, you and I couldn’t see and hear the same bee. But we can see and hear the same bee, so it seems that the bee can’t be a particular idea either. So what then is the bee? I think the core of Berkeley’s answer is that ordinary objects are collections of ideas. You and I can see the same bee because we can experience different particular ideas that belong to the same collection of ideas that is the bee.

There is a standard objection to Berkeley’s answer. We normally suppose that ordinary objects exist when no one perceives them. It’s odd to suppose that our bee pops into existence when I perceive it, then pops out of existence when I cease to perceive it, then pops back into existence when you perceive it, then pops out of existence again when you stop perceiving it.

Commentators have long disagreed over how Berkeley means to respond to this objection. My own view is that Berkeley’s response is best thought of as having two sides. On the one side, we find a bit of principled insouciance. Most of us are not idealists, and so it strikes us as a big deal that ordinary objects shouldn’t have “gappy” existences. It seems utterly implausible to us that, say, the moon or Mount Rushmore should pop out of existence when no one is looking and pop back into existence when the park reopens or someone goes for a midnight stroll. As an idealist, however, Berkeley needn’t be – and in fact isn’t – all that concerned about the continuous persistence of ordinary objects. Having already identified ordinary objects with collections of ideas, I don’t think he personally finds the possibility that ordinary objects might pop in and out of existence all that troubling.

On the other side, however, Berkeley does recognize that most people will think that ordinary objects persist continuously, and he thinks that such a belief is at least consistent with his idealism. The bee might continue to exist even when we don’t perceive it because God is always there to perceive it when no one else does. Alternatively, or in addition, the bee might be said to persist even when we don’t actually perceive it because certain counterfactuals about it are true. Supposed for example that the bee flies into a hive where no one can see nor hear it. We might say that the bee nonetheless continues to exist provided that it is still true that if we were to break open the hive we would see and hear the bee.

3:AM:You’ve also thought about Berkley’s views on human agency and divine concurrentism. So first of all, what are the options for dealing with this that Berkeley might have chosen? What’s the philosophical puzzle concurrentism raises?

JM:Already in the Middle Ages, philosophers had confronted the question of how the activity of creatures – including the activity of humans – relates to God’s activity. Essentially everyone agreed that creaturely activity depends upon God’s creating creatures. If creatures aren’t created, they can’t do anything. Essentially everyone also agreed that creaturely activity depends upon God’s conserving creatures. The thought here is that just as creatures couldn’t begin to exist without God’s creating them, so too they couldn’t continue to exist without God’s sustaining them in existence. And, again, if creatures don’t exist, they can’t do anything. That’s a lot of dependence already, but most medieval philosophers thought that creaturely activity depends upon divine activity in one further way: they maintained that God’s active assistance was necessary for creaturely activity to be causally efficacious. On this view, one billiard ball can cause another billiard ball to move only if God actively assists the first billiard ball in its activity. In Scholastic parlance, one billiard ball can cause another billiard ball to move only if God concurs with the first billiard ball’s activity.

Thinking about the relationship between creaturely and divine activity raised two philosophical puzzles in particular. The first is rooted in the relationship between creaturely activity and God’s sustaining activity – what Scholastics called divine conservation. Most Scholastics maintained that God’s conserving activity is intimately related to his creating activity, and, indeed, they generally maintained that conservation is a continued creation. But what does that mean? One thing that it might mean – and I think this is what it did mean for most Scholastics – is that God’s creation and conservation are best thought of as one continuous activity, with the term “creation” indicating the beginning of that activity, and the term “conservation” indicating the continuation of that activity. In a slogan: conservation is continued or continuous creation. But another thing that it could mean is that to conserve a creature at a time just is to create – or better, to recreate - that creature at that time. In a slogan: conservation is a continuous recreation.

This puzzle had surprisingly far-reaching implications since many philosophers found it hard to see how creatures could do anything if God were recreating them at every instant. How can one billiard ball at this instant cause another billiard ball to move at the next instant if both must be created entirely anew at every instant? I think things look less dire, however, if conservation is a continuous creation. If conservation is a continuous creation, we might think of creation and conservation as being necessary for the existence of creatures in a way that is perfectly consistent with their causally interacting with one another. We might think of God’s creating and conserving creatures as being a slightly analogous to a freezer’s creating and conserving ice cubes. The freezer’s creating and conserving ice cubes seems obviously consistent the ice cubes themselves enjoying causal powers – an ice cube might, for example, support another ice cube or prop up a half-eaten tub of ice cream.

A second puzzle is rooted in questions concerning how exactly divine and creaturely concurrence might work. The whole point of the doctrine of divine concurrence is that creatures can’t bring about anything without God’s assistance. In that case, however, it seems like divine concurrence can’t be akin, for example, to one person helping another person to lift a heavy weight that she couldn’t lift on her own. For that model implies that the person being helped could have lifted a lighter weight by herself.

I think we might try to think of divine concurrence as being a little like the operation of electrical appliances. When a toaster toasts a piece of bread the toaster is a necessary but not sufficient condition for the toasting. It needs the help of the electricity that powers it. But the electricity is likewise a necessary but not sufficient condition for the toasting. It needs the help of the toaster. Something a little like this is going on in the case of divine concurrence. According to proponents of divine concurrentism, when I raise my hand, my willing to raise my hand is a necessary but not sufficient condition for my hand going up, and God’s particular act of concurrence is likewise a necessary but not sufficient condition for my hand going up.

3:AM:So what did Berkeleydo? Is his approach different from, say, Malebranche’s occasionalism, or Leibniz’s solution?

JM:In a striking entry in his notebooks, Berkeley makes a remark to the effect that a person without God would be more wretched than a stone, having, as he puts it, “only the power to be miserable by his unperformed wills, these having no power at all.” Inspired in part by this remark, as well as by what they see as pressures inherent in his philosophical system, some commentators have held that Berkeley is – like Malebranche – an occasionalist. That is, they have thought that Berkeley, like Malebranche, held that God is the only genuine cause and that creatures serve merely as “occasions” for God’s direct causal intervention in the world. In another entry in his notebooks, however, Berkeley also writes that “we move our legs ourselves” and that in holding this view he differs from Malebranche. That, of course, presents a difficulty for occasionalist readings and suggests that Berkeley might not be so similar to Malebranche after all.

I think the best way of making sense of both passages, and of Berkeley’s position more generally, is to read him not as an occasionalist but rather as a concurrentist. Without God, we would indeed be incapable of bringing about any effects. We’d be unplugged toasters as it were. But that doesn’t entail the occasionalist conclusion that we don’t causally do anything in the world. We might still contribute to the production of effects with God’s concurrence, that is, with God’s assistance. We might be plugged in toasters.

So I do think Berkeley differs from Malebranche, and I think his position has some real advantages over Malebranche’s position. The deepest problem confronting Malebranche’s occasionalism is to make sense of human responsibility. It’s hard to see how we can be responsible for anything if we are not causally involved even in the production of our own volitions or thoughts. Insofar as they both embrace concurrentism, Leibniz’s position is much closer to Berkeley’s than Malebranche’s. Indeed, if we focus specifically on the production of volitions, I think Leibniz’s and Berkeley’s views are actually quite similar.

3:AM:Kant attempted to refute Descartes’ skepticism. You’ve won a prize for defending the Kantian refutation. What’s at stake here, and why do you think Kant’s refutation still has things to say to contemporary readers?

JM:In the second edition of the Critique of Pure Reason, Kant inserted a short section known as the “Refutation of Idealism.” There, Kant offers an intriguing but difficult argument that a particular brand of external world skepticism is incoherent or unsustainable. The skeptic Kant has in mind maintains that she has a rich knowledge of her own mental life but lacks knowledge of the external world. She knows that it seems like she woke up this morning, then went for a run, and is now enjoying lunch. But, she claims, she doesn’t know that she’s not in fact in bed or even that she’s not in fact a disembodied soul being tricked by an evil demon.

On an interpretation of the Refutation developed by Paul Guyer, Kant argues that the skeptic’s position is incoherent because her rich knowledge of her own mental life presupposes her having knowledge of the external world. If Kant is right, the skeptic couldn’t have robust knowledge about the order of her own mental states without having some knowledge about an external world. In short, rich self-knowledge presupposes external world knowledge.

Kant’s overarching thought is that knowledge in general presupposes knowledge of lawful regularities. He insists, however, psychological laws are impossible. But, if there can’t be psychological laws, then knowledge in general will require knowledge of the external world in order to have knowledge of lawful regularities. Without knowledge of the external world, we couldn’t have knowledge of any laws, and without knowledge of any laws, we can’t have rich knowledge even of our own mental lives.

On Guyer’s interpretation, Kant’s anti-skeptical argument appears to turn crucially on his view that psychological laws are impossible. Unfortunately, not too many people have been convinced by Kant’s arguments against the possibility of psychological laws. I’ve argued that Kant can effectively sidestep this metaphysical issue. The skeptic’s position rests on a certain conception of knowledge, a conception according to which something can count as justifying our beliefs only if we have knowledge of that thing. On the skeptic’s conception of knowledge, the fact that I have hands can count as justifying my belief that I’m not a brain in a vat only if I know that I have hands. If that’s right, however, Kant’s argument doesn’t turn on psychological laws being impossible, but rather only on our not having knowledge of psychological laws. And that, I think, is a much easier premise to defend.

Whatever one thinks of Guyer’s interpretation, or my development of it, Kant’s subtle argument still has much to teach contemporary readers, and I find his suggestion that self-knowledge is more demanding than it might at first seem especially intriguing.

3:AM:How does asking why it is absurd to suppose that Julius Caesar is a number helpful in developing a non-spooky mind-body dualism? I had to ask!

JM:You mean it isn’t obvious? (Just kidding of course.) The question of why it’s absurd to suppose that Julius Caesar is a number goes back to the father of modern logic, Gottlob Frege. Frege thought that we could get some insight into what a number is by considering why it is so obvious to us that numbers can’t be people. Why couldn’t the same thing quanumber numerate the planets and quaperson cross the Rubicon? Why couldn’t the very same thing have two totally different sets of properties, in this case numerical properties and physical properties?

In a paperI wrote with John Hawthorne, we tried to show how answers to Frege’s question might be carried over to questions in the philosophy of mind. The question of why the mind can’t be identical to the body is analogous to the question of why Caesar can’t be identical to the number 9. More specifically, if you think that minds and bodies have two totally different sets of properties you might see considerations arising from Frege’s question as pushing you in the direction of a full-blown substance dualism. Just as we don’t think that one thing could have both numerical and physical properties, we might think that one thing could not have both mental and physical properties.

After arguing that reasonable answers to Frege’s question suggest that property dualists should also be substance dualists, we go on to argue that substance dualism itself needn’t be especially “spooky.” Consistent with our arguments, a substance dualist can maintain that minds are realized by physical bodies, that is, that while minds can’t be identical to brains, minds might nonetheless be metaphysically grounded in brains.

3:AM:And finally, are there five books that you could recommend to the curious readers here at 3:AM that would take us further into you philosophical world?

JM:Gladly!

G. W. Leibniz, Philosophical Essays, ed. and trans. Daniel Garber and Roger Ariew (Hackett Publishing Company).

George Berkeley, Three Dialogues between Hylas and Philonous, ed. Jonathan Dancy (Oxford University Press).

Maria Rosa Antognazza, Leibniz: An Intellectual Biography(Cambridge University Press).

Daniel Garber, Leibniz: Body, Substance, Monad(Oxford University Press).

Donald Rutherford, Leibniz and the Rational Order of Nature(Cambridge University Press).

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