The Mind-Body Problem 3.0

Fabrizio Calzavarini • Marco Viola Editors Neural Mechanisms New Challenges in the Philosophy of Neuroscience Chapter 12 The Mind-Body Problem 3.0 Marco J. Nathan Abstract This essay identifies two shifts in the conceptual evolution of the mind- body problem since it was molded into its modern form. The “mind-body problem 1.0” corresponds to Descartes’ ontological question: what are minds and how are they related to bodies? The “mind-body problem 2.0” reflects the core issue underlying much discussion of brains and minds in the twentieth century: can mental states be reduced to neural states? While both issues are no longer central to scientific research, the philosophy of mind ain’t quite done yet. In an attempt to recast a classic discussion in a more contemporary guise, I present a “mind-body problem 3.0.” In a slogan, this can be expressed as the question: how should we pursue psychology in the age of neuroscience? 12.1 Introduction The “mind-body problem”—the hallowed task of characterizing the relation between the mental and the physical—lies at the core of the philosophy of mind. Still, its nature remains baffling. What exactly makes it a problem? What would constitute a viable solution? When did the issue arise? How did it evolve over time? And why is it still troubling after all these years? The mind-body problem is typically presented as a single, monolithic, perduring puzzle that has framed discussions of mental states, at least, since Descartes molded the question into its current form.1 This essay examines, and, ultimately, rejects 1 Itis not trivial to find explicit statements of this assumption, partly because the mind-body problem is well-known and contemporary authors seldom bother to present it in full detail. Here are some representative quotes: “[T]he persuasive imagery of the Cartesian Theater [the idea of a centered locus of consciousness in the brain] keeps coming back to haunt us—laypeople and M. J. Nathan (!) University of Denver, Denver, CO, USA e-mail: [email protected] © Springer Nature Switzerland AG 2021 263 F. Calzavarini, M. Viola (eds.), Neural Mechanisms, Studies in Brain and Mind 17, https://doi.org/10.1007/978-3-030-54092-0_12 264 M. J. Nathan this presupposition. Over time, the content of the mind-body problem has shifted substantially. The inquiries driving contemporary philosophy of mind are not the original ones troubling Descartes. This point is not especially original, as prominent scholars such as Kim (1999, 2011) and Heil (2013), have advanced analogous points. It should also not come as a real shock, given that almost four centuries have passed since the publication of the Meditations, in 1641. More controversially, I suggest that twenty-first-century research has moved away from the theoretical discussions that framed the interface between psychology and neuroscience just a few decades ago. Thus, on the widespread assumption that philosophy and science do—and ought to—mutually inform one another, the mind-body problem requires a makeover. It is time to update our philosophical agenda. This article is structured as follows. §2 kicks off the discussion by introducing what I call the “mind-body problem 1.0.” This is Descartes’ ontological question: what are minds and how are they related to bodies? After briefly surveying Descartes’ well-known proposal, its shortcomings, and the main alternatives, I conclude that this issue was never solved. Rather, it was “dissolved,” that is, recast in a related but different form, when people realized that neither substance monism nor substance dualism tell us much about the nature of mind. This reformulation, which I call the “mind-body problem 2.0,” is presented in §3. The mind-body problem 2.0, simply put, is the core issue underlying much discussion of brains and minds in the century just passed: can mental states be reduced to neural states? Just like version 1.0, the mind-body problem 2.0 is no longer central to twenty-first-century scientific research. The main culprit, I maintain, is the lack of a clear and coherent framework for characterizing reduction. My argument consists of two main steps. First, §4 provides a succinct overview of how reduction has been conceived in the philosophy of science, since the “classical” model of the 1960s. Second, §5 maintains that it is time to move away from questions of reduction, which are less substantive, more terminological than it is often assumed. Similar observations have triggered provocative proclamations of the philosophy of mind being over. Such obituaries strike me as premature. Philosophy of mind ain’t quite done yet. In an attempt to recast the traditional heart of the subfield, the mind-body problem, in a more contemporary guise, §6 poses a “mind-body problem version 3.0.” In a slogan, this can be expressed by the question: how should we pursue psychology in the age of neuroscience? Finally, §7 wraps up the discussion with concluding remarks. Before moving on, a few preliminary clarifications are in order. First, the discus- sion in the ensuing pages admittedly presupposes a modest form of methodological naturalism, according to which philosophical and scientific analyses are mutually relevant. Critics who view philosophy as a purely “armchair” intellectual endeavor, scientists alike—even after its ghostly dualism has been denounced and exorcized” (Dennett 1991, p. 107). “The mind-body problem was posed in its modern form only in the seventeenth century, with the emergence of the conception of the physical world on which we are now all brought up” (Nagel 1995, p. 97). “What exactly are the relations between the mental and the physical, and in particular how can there be causal relations between them? ( . . . ) This is the most famous problem that Descartes left us, and it is usually called the ‘mind-body problem”’ (Searle 2004, p. 11). 12 The Mind-Body Problem 3.0 265 insulated from empirical observations, will be likely left unmoved. Second, at the same time, my goal is not to eschew philosophical problems and replace them with scientific ones. My aim is rather to show how classic philosophical problems, appropriately revamped, are still quite pertinent to empirical inquiries. Third, and relatedly, some readers may wonder about the advantages of characterizing modern ventures into the philosophy of psychology and neuroscience as variants of the old “mind-body problem.” Once we recognize that we have moved away from Cartesian concerns, why not dismiss the mind-body problem as a historical relic of a bygone time? My response, in brief, is that the overarching moniker provides a useful guideline to appreciate the historical continuity across the field. Even though version 3.0 is different from both 2.0 and 1.0, treating them a family of issues pertaining to the relation between the mental and the physical at large helps us see how each problem rises from the ashes of its predecessor. Fourth, and finally, although much of the ensuing discussion covers well-known terrain, the overarching aim of this essay is not merely, or even primarily, expository. My goal is to provide a critical diachronic overview and a fresh diagnosis of past issues. This rational reconstruction suggests an alternative trajectory for the future of the philosophy of mind. 12.2 The Mind-Body Problem 1.0 Our journey begins by revisiting an old story. This is the tale of how Descartes provided the original formulation of the modern mind-body problem, setting the stage for subsequent discussions over the centuries to come. Descartes lived most of his life in the seventeenth century, a time of profound change across the sciences. Setting nuances aside, natural philosophy was in the pro- cess of moving away from the teleological worldview inherited from Aristotle and subsequently developed by medieval scholastics, heading towards the mechanistic Weltanschauung pioneered by Galileo. Descartes, who was a fine man of science, enthusiastically endorsed the in-principle possibility of subsuming the physical universe under deterministic laws, eschewing any reference to goals, purposes, or other forms of teleology. At the same time, as a deeply religious and moral man, Descartes was troubled by the thought that humans might be nothing more than complex machines. Some readers might feel inclined to brush off Descartes’s qualms with uncom- promising materialism as a legacy of a bygone time, a pernicious combination of religious dogmatism and factual ignorance. Yet, such interpretation would be both uncharitable and inaccurate. First, from a historical perspective, Descartes was very much on top of the science of his time, as witnessed by his notable contributions to various fields, such as mathematics, physics, and physiology. Second, from a conceptual standpoint, Descartes’ rationale for eschewing radical physicalism was hardly antiscientific. Simply put, he realized that the behavior of conscious and unconscious entities is not explained in the same way. Inanimate objects typically obey strict physical equations or mechanistic law-like generalizations. 266 M. J. Nathan Animate organisms, in contrast, are subsumed under intentional, goal-directed, or teleological descriptions, such as those commonly found in current psychology, sociology, economics, and related fields. The psychological explanation of an agent pouring herself a glass of water because she intends to quench her thirst looks nothing like the mechanistic account of why a glass shatters when it falls to the ground. This discrepancy, no less evident today than it was in the 1600s, raises obvious follow-ups. What underlies the difference? What exactly distinguishes animate organisms from inanimate objects? Descartes’ proposal is so famous that a few brief remarks should suffice. Human beings, he claimed, are not purely material. We have both extended bodies and minds. Given our res cogitans plus res extensa composition, our behavior will be the resultant of mental and physical causes. Then what characterizes these substances? Do they interact? If so, how? This was the birth of the mind-body problem or, more precisely, what I call the “mind-body problem 1.0.” Descartes’ concerns were primarily ontological. Mental states cannot be analyzed physically because they are not material things at all. A mind, for him, is a sui generis kind of substance: res cogitans.2 In the 1600s, the ontology of mind was truly an open issue. Interactionism was hardly an ad hoc stipulation. It was a fecund speculative hypothesis. Sure, Elizabeth of Bohemia was quick to pinpoint troubling aspects. Yet, after Descartes’ Meditations, Hobbes, Spinoza, Malebranche, Leibniz, and other prominent philoso- phers and scientists debated whether there is a substance, a force, an élan vital distinguishing animate from inanimate entities. Things changed. By the late 1800s, empirical evidence against substance dualism had rapidly mounted. With the eclipse of vitalism in biology, Descartes’ research program regressed and was eventually replaced by forms of substance monism, which became the default ontology against which to address psycho-physical relations and related methodological issues. By the mid-1900s, most scholars viewed minds either as physical systems or as being realized by such systems. As we’ll see, this includes authors with Cartesian inclinations, who replace substance dualism with alternative frameworks, such as property dualism or panpsychism. The vast majority of scientists and philosophers found the case against res cogitans overwhelming. This suggests that Descartes’ original question, “mind-body problem 1.0,” has finally been answered. In a sense, it has. Minds are no longer characterized as ontologically distinct. Yet, rejecting res cogitans evidently tells us little about the nature of mind. Substance monism, alas, leaves ample room for disagreement regarding which properties constitute or instantiate mental states. In particular, it does not constrain how psychological systems must relate to their physical substrate. In this other sense, Descartes’ problem was never solved. It was dissolved, recast in a related albeit novel guise. 2 Descartes’s conception of substance was strikingly nuanced (Rodriguez-Pereyra 2008). 12 The Mind-Body Problem 3.0 267 This conceptual shift can be clearly seen in mid-twentieth century philosophy of mind. Ryle (1949, pp. 21–22) famously wrote that “[Descartes] had mistaken the logic of his problem. Instead of asking by what criteria intelligent behaviour is actually distinguished from non-intelligent behaviour, he asked ‘Given that the principle of mechanical causation does not tell us the difference, what other causal principle will tell it us? He realized that the problem was not one of mechanics and assumed that it must therefore be one of some counterpart to mechanics.” Ryle’s reconstruction is accurate. Except that Descartes was hardly mistaken about the logic of his problem. Rather, he was raising an issue about ontology. Ryle and many of his Anglo-Saxon colleagues, in contrast, had an altogether different question in mind. 12.3 The Mind-Body Problem 2.0 The previous section surveyed Descartes’ groundbreaking speculations regarding the relation between the mental and the physical. Descartes’ concern was, first and foremost, an ontological one. Given that minds cannot be physical entities, he wondered, what kinds of substance could they be? His query was answered, once and for all, at the turn of the twentieth century, when vitalism was expunged from biology and most scholars, scientists and philosophers alike, embraced forms of substance monism. Still, and this is the crucial point, settling the issue revealed very little about the nature of mind. What kind of physical systems are minds or instantiate them? How should mental states be studied? What distinguishes conscious organisms from inanimate objects? To address these questions, which lie at the heart of the philosophy of mind, Descartes’ problem—the mind-body problem 1.0— had to be reformulated. How, exactly? To guide our discussion, let’s look at some influential theories of mind articulated in the century just passed. One of the first notable materialist theories of mind developed in the twentieth century, popular among both philosophers and psychologists, is behaviorism, which analyzes mental states as complex clusters of dispositions to behave.3 A few years later, the type-identity theory, pioneered by U.T. Place (1956) and J.J.C. Smart (1959), purported to identify mental states and brain states at the type level. In response to objections to type-identity theory, especially those pertaining to the multiple-realizability of psychological kinds (Putnam 1967; Fodor 1974), 3 To be sure, psychologists and philosophers had different agendas. To reflect this divergence, it is common to distinguish two strands of behaviorism (Fodor 1981). First, “philosophical” (also known as “logical” or “analytic”) behaviorism is associated with a thesis about the nature of mind and the meaning of mental states. Second, “psychological” or “methodological,” behaviorism emerged from an influential scientific methodology applied to psychology. For the sake of simplicity, I shall not distinguish between the two variants. 268 M. J. Nathan functionalism refined behaviorist insights by characterizing mental states as causal roles determined by inputs, outputs, and various other kinds of internal connections with other mental states (Putnam 1965; Armstrong 1981).4 An alternative path was explored by Donald Davidson (1970), who posited an identity of mental and physical states at the token level—whence the name token-identity theory of mind— and described their relation in terms of supervenience. Another view is eliminative materialism, a position principally advocated by Patricia and Paul Churchland (1981, 1986), which treats mental states as theoretical entities posited by folk psychology. Commonsensical as it may seem, they argue, folk psychology is a flawed theory of mind and, as such, it is not a candidate for integration. Rather, it should be eliminated and replaced by a mature neuroscience, which will turn out to be more predictive, explanatory, and connected to other fields of science. A final noteworthy approach is a revamped version of dualism. Property dualism agrees with Descartes that there is a real distinction between mental and physical attributes. Yet, it does not view res cogitans and res extensa as mutually exclusive. One and the same substance may have both physical and mental properties. While substance dualism today has few, if any, proponents, property dualism is still advocated in the philosophical literature (Jackson 1982; Chalmers 1996). A comprehensive overview of these, and other, influential theories of mind lies beyond the scope of this essay. For present purposes, the crucial matter— pun intended—is pinpointing the main matter of contention underlying all these approaches. As soon as this is done, it immediately becomes evident how irrelevant Descartes’ question, the mind-body problem 1.0, has become. The main reason is not that most parties involved, including property dualists, eschew substance dual- ism. This is true, but of marginal significance from our contemporary standpoint. After all, even some of Descartes’ contemporaries questioned his ontology. Whether minds are mental substances or physical substances has become utterly tangential to the twentieth-century debate. The modern focus is on the issue of reduction. Can mental states be reduced to brain or, more generally, physical states? If so, how? If not, why not? Behaviorism, type-identity theory, and eliminative materialism all answer in the positive: psycho-neural reduction is feasible, at least as a matter of principle. Token-identity theory, functionalism, and property dualism answer in the negative, claiming that any such reduction is doomed to failure.5 In short, the in- principle reducibility of mental states to physical states, or the impossibility thereof, 4 Again, I am unabashedly clashing together several variants of functionalism, such as Putnam’s “psycho-functionalism” and Armstrong’s “a priori functionalism” (Block 1978). 5 As Matteo Colombo has brought to my attention, the mind-body problem 1.0 could also be framed as a matter of reduction. On this reading, Descartes may be interpreted as providing a negative argument: minds cannot be reduced to bodies because they are altogether different substances. This is an effective strategy to bring Descartes into modern debates, finding some narrative continuity in the last four hundred years of philosophy of mind. Still, this operation should be understood, from our contemporary perspective. From historical standpoint, Descartes’ target was not reduction. He was interested in ontological questions about the nature of minds and their interactions with bodies. 12 The Mind-Body Problem 3.0 269 lies at the core of twentieth-century philosophy of mind. This is what I call the “mind-body problem 2.0.” If mind-body 2.0—that is, psycho-neural reduction—is so central to contempo- rary philosophy of mind, one could legitimately ask, what can be said about its success or failure after decades of extensive debate? Even a cursory look at the specialized literature reveals that clear-cut, conclusive answers are nowhere to be found. Of course, we have made significant progress in the discovery of psycho- neural mechanisms underlying higher and, especially, lower cognition. But have these findings advanced the tout court reduction of mental states to brain states? If so, the news has not been broken, as there seems to be no more consensus today than there was in the 1950s. When confronted with this lack of resolution, many scientists and philosophers interested in the nature of mental states tend to justify the situation by appealing to the intricacy of the subject matter. The human brain is the most complex organic structure discovered so far in the universe, composed of billions of cells and an astronomical number of possible connections among them. No wonder that solving the dispute is so darn hard! I have no quibbles with any of the premises. Studying the human brain is, indeed, frustratingly difficult. Nevertheless, I am skeptical of the diagnosis. The complexity of the structures under investigation is not the principal cause of the lack of tangible progress when it comes to the mind-body problem 2.0. The main culprit, as I will go on to argue, is the notion of reduction itself. Before moving on, I should clarify what distinguishes my view from similar positions in the literature. Over the last few decades, there has been no shortage of philosophical attempts to explain away the mind-body problem. Notably, Chomsky (2000, 2002) has written extensively on the topic, arguing convincingly that contrary to common wisdom, the mind-body problem “did not disappear because of inadequacies of the Cartesian concept of mind, but because the concept of body collapsed with Newton’s demolition of the mechanical philosophy” (2002, p. 71). I am, indeed, quite sympathetic to Chomsky’s remarks. Yet, I want to draw attention to a different issue, that applies not much to Descartes’ original position—the “mind-body problem 1.0”—but to question of the reducibility of mental states. This corresponds to what I call the “mind-body problem 2.0.” It should be evident how my attempt to undermine the very question of reduction puts me at odds with both traditional reductionist and antireductionist philosophical perspectives. Spelling out the argument involves breaking it down into two main steps. First, §4 takes a detour into the history of the philosophy of science, focusing on how the concept of reduction has morphed since the collapse of the classic model of reduction, endorsed by logical positivism. Next, §5 explains why focusing on the issue of reduction—the crux of the mind-body problem 2.0—might have been a red herring driving philosophers down a wrong path. Relatedly, eliminative materialists prefer to talk about “elimination” as opposed to “reduction.” Yet, the former concept can be straightforwardly treated as a limiting case of the latter. 270 M. J. Nathan 12.4 Pegs, Holes, Atoms: An Overview of Reduction What does it mean to “reduce” a theory, a concept, or a law of nature? Positivist philosophy of science had a clear, unequivocal answer, whose locus classicus became Nagel’s (1961) The Structure of Science. Simply put, from the standpoint of logical empiricism, to reduce a theory T1 to a theory T2 is to show that the laws of T1 can be deduced from the laws of T2 via “bridge principles” that translate the concepts of T1 into the vocabulary of T2. 6 It is worth stressing that reductionism, as constructed by Nagel, is ontologically neutral, in the sense that it is independent of physicalism or dualism. Now, most philosophers of science since the mid-twentieth century, friends and foes of positivism alike, have been thoroughgoing materialists. Nevertheless, Cartesians too could conceive of reduction—or, presumably, lack thereof—along Nagel’s lines. Again, this goes to show how much the debate had shifted from Descartes’ original formulation of mind-body 1.0. Influential as it was, Nagel’s reductionism was eventually eroded by powerful objections. Setting details aside, the main problem involved the lack of bridge laws. With the possible exception of a few hackneyed examples, the multiple-realizability of kinds across the special sciences implies that there are not enough connecting principles for classical derivational reduction to take flight as a general model of science (Putnam 1967; Fodor 1974). Does this mean that the mind-body problem 2.0—the question of psycho-neural reduction—has finally been answered in the negative? Not at all. The collapse of classical reductionism hardly signaled the end of reductionism tout court. The old positivist model has been reformulated and replaced by a novel, more promising framework, intended to avoid the shortcomings of its illustrious predecessor. The new wave of reductionism is packaged as a set of epistemic questions concerning explanation. Can we provide micro-depictions of all macro-events? And are the resulting lower-level explanations invariably deeper than their higher-level coun- terparts? Reductionists typically answer both questions in the positive, advocating a form of epistemic fundamentalism. Whereas it is often epistemically necessary or pragmatically convenient to stick to macro-descriptions, the neo-reductionist story goes, adding detail always increases the depth of coarser descriptions. Readers will likely guess my follow-up. Can we describe every scientific event at more fundamental, finer-grained levels? And it is really the case that these micro-depictions invariably enhance explanatory power? Despite valiant attempts to resolve the conundrum, clear-cut answers are still wanting. Why is this so? The main reason, as we’ll see in §5, involves the vagueness of reduction. Despite the appearance of substantive disagreement, both parties end up talking past each other. 6 To be sure, Nagel’s own conception of reduction was subtler, and its proper interpretation remains a matter of controversy (Fazekas 2009; Klein 2009). Nevertheless, for present purposes I am less interested in Nagel’s actual views, and more in how his model of reduction was received and discussed within philosophy (Fodor 1974; Kitcher 2003). 12 The Mind-Body Problem 3.0 271 In order to get there, however, we need to continue to follow the unravelling of this longstanding debate. In an influential article published over four decades ago, Putnam (1975) argued that traditional discussions of the mind-body problem rest on a misleading assump- tion. The problematic presupposition in question is a conditional premise: if we accept that human beings are purely material entities, then there must be a bona fide physical explanation of our behavior. Physicalists, Putnam noted, use this premise in a modus ponens inference: – (a) Humans are purely material beings. – (b) If humans are purely material beings, then there must be a bona fide physical explanation of our behavior. – (c) ∴ There must be a physical explanation of our behavior. Dualists, in contrast, embed the conditional (b) in a modus tollens inference, rephrased here in the subjunctive mood, to enhance readability: – (b) If humans were purely material beings, there would be a bona fide physical explanation of our behavior. – (c*) There is no physical explanation of our behavior. – (a*) ∴ Humans are not purely material beings. These two arguments advance diverging conclusions. Yet, physicalists and dualists alike accept the conditional premise. This, Putnam maintains, is a mistake. Both parties miss the mark, as (b) should be rejected as unsound. In support of his conclusion, Putnam presents a suggestive analogy (Fig. 12.1). He considers a rigid board with two holes: a circle exactly one inch in diameter and a square one inch high. Now, take a cubical peg just under one inch high. The peg will go through the square hole. However, it will not go through the round hole. How do we explain these elementary observations? Putnam sketches two types of explanations. The first begins by observing that the board and the peg are rigid lattices of atoms. If we compute the astronomical number of all physically possible trajectories of the peg, we will eventually discover that no trajectory passes through the round hole, whereas at least one trajectory, likely more, passes through the square hole. An alternative explanation begins in exactly Fig. 12.1 Putnam’s square-peg-round-hole example 272 M. J. Nathan the same way, by noting that the board and the peg are rigid systems. Yet, instead of comparing trajectories, it points out that the square hole is slightly larger than the cross section of the peg, whereas the round hole is smaller. Call the former kind of explanation “physical,” “lower-level,” or “micro” and label the latter one “geometrical,” “higher-level,” or “macro.” The question is: are both explanations adequate? If not, why not? And, if so, which one is better and why? Putnam contends that the geometrical explanation is objectively superior. (Actu- ally, Putnam goes as far as claiming that the physical explanation is not explanatory at all, but I set this more controversial thesis to the side.) The reason is that, whereas the physical description only applies to the specific case at hand, the geometrical story generalizes to similar structures. To illustrate, an exhaustive listing of all trajectories will only account for why this peg will or will not go through these particular holes. In contrast, the geometrical account captures why no square peg will go through a hole smaller than its cross-section. As Putnam (1975, p. 297) puts it, “in terms of real life disciplines, real life ways of slicing up scientific problems, the higher-level explanation is far more general, which is why it is explanatory.” The significant philosophical moral drawn by Putnam from this intuitive toy example is the explanatory autonomy of the mental from the physical. Higher-level explanations, regardless of whether they involve pegs and holes, or psychological states cannot—and should not—be explained at lower levels, in terms of neurosci- entific, biochemical, or physical properties. Putnam’s argument has left a mark by firing up a longstanding debate. Philoso- phers started asking: is it really the case that macro-explanations are objectively superior to their micro-level counterparts? Antireductionists answered in the pos- itive. In the philosophy of mind, authors such as Fodor (1968), Davidson (1970), Jackson (1982), Yablo (1992), Chalmers (1996), Hornsby (1997), and Burge (2007, 2013) have buttressed various arguments supporting the autonomy of mental states from underlying neural ones. Reductionists beg to disagree. Scholars like Paul and Patricia Churchland (1981, 1986), Bickle (1998, 2003), and Kim (1999) have countered that micro-explanations are the key to deepen our understanding of the mind.7 Obviously, at the most general level, the question of reduction must be under- stood as a matter of principle, not practice. Current physics is not even close to replacing biology, psychology, economics, or any other special science. We lack the understanding of subatomic systems and, especially, the computing power required to approximate the perfect vision of a “Laplacian Demon.” Still, reductionists claim, in theory, it would be possible to provide micro-explanations to replace 7 An analogous, equally heated debate emerged in the philosophy of science. Putnam’s square- peg example was developed and extended to real-life scientific scenarios in biology (Kitcher 2003), psychology (Fodor 1974), and the social sciences (Garfinkel 1981). Post-positivist neo- reductionists disagreed. Authors such as Waters (1990), Sober (1999, 2000), Rosenberg (2006), and Strevens (2008) stressed that, while micro-explanations are often unnecessarily complex or anti-economical, they do emphasize crucial details that are typically presupposed implicitly or taken for granted at the macro-level. 12 The Mind-Body Problem 3.0 273 and improve current macro-depictions. Antireductionists reject this conclusion. Addressing biology, psychology, or economics in physical terms, they argue, would not deepen these inquiries. In short, Putnam revamped the debate on epistemic reductionism across the sciences by questioning the possibility of explaining higher-level states at more fundamental levels. Fellow antireductionists follow suit and embrace the explana- tory autonomy of the mental, motivated by structurally analogous arguments. Contemporary reductionists retort that these considerations miss the mark. Lower- level explanations, they suggest, always enhance the explanatory power of coarser depictions. But was this the right direction to point the discussion? Is explanatory reduction the core issue underlying the square-peg-round-hole scenario and its implications for the philosophy of mind? As we shall now see, there are reasons to be skeptical. 12.5 Some Bugs in the Mind-Body Problem 2.0 Let’s take stock. §4 retraced the origins of reduction, the conceptual core of the mind-body problem 2.0 and of much discussion in twentieth-century philosophy of mind. The issue driving the debate is whether breaking down macro-explanations into micro-explanations invariably increases explanatory power. Epistemic reduc- tionists argue in the positive. Antireductionists answer in the negative. How much progress have we made towards a solution? To get started, consider the current state of psycho-neural reduction. While recent advancements in cognitive neuroscience have yielded a plethora of results, much remains unknown. Reductionists typically stress the remarkable successes with sensory systems and various domains of lower cognition such as early vision, pain, and taste, as evidence for the power and promise of decomposition strategies. Antireductionists rejoin that comparable achievements cannot be boasted for language processing, decision making, and other domains of higher cognition, especially consciousness. Despite this divergence, both parties agree that knowl- edge of the structure and location of psycho-neural mechanisms implementing and computing cognitive functions has increased exponentially. The philosophical debate hinges on whether or not it is possible to enhance the power of higher-level explanations via lower-level descriptions. This dichotomy, note, mirrors Putnam’s square peg round hole scenario. But is this the proper analogy to draw? To assess the prospects of psycho-neural reduction, understood along the lines just delineated, it is instructive to compare it with the corresponding debate over reductionism in the life sciences. This mirroring is enlightening because neural mechanisms are still relatively obscure, due to the complexity of the system under study. In contrast, biologists have a clearer picture of the implementation of functional structures at the molecular level. We already know quite a bit about, say, how important phylogenetic adaptations are transmitted across generations and develop at the ontogenetic level. 274 M. J. Nathan These considerations suggest that the case for or against reductionism is closed, or is close to being settled, in the life sciences. After all, if the crucial issue is whether all macro-biological explanations can be strengthened at the micro- biological level, having concrete case studies to assess should provide decisive evidence, one way or the other. To be sure, the fate of reductionism tout court depends on much more than a handful of successful or failed stories. Even the accomplished reduction of, say, evolution to molecular genetics would still fall short of an overarching reductionism. Nevertheless, it would provide strong evidence in favor of reductionism as a “working hypothesis.” Unfortunately, the jury is still out, and any verdict is far from reached. The status of epistemic reductionism in genetics, ontogeny, evolution, and other branches of biology remains as open and controversial as ever (Sarkar 1998; Sober 2000; Kitcher 2003; Rosenberg 2006; Dupré 2012; Griffiths and Stotz 2013). Sophisti- cated antireductionists acknowledge the success of molecular biology. Still, they stress how so-called “molecular” explanations consistently appeal to structural and functional concepts, and holistic states of systems. This, antireductionists claim, shows that the appearance of reduction is nothing but a smoke screen. Modest reductionists, in contrast, appreciate the importance of functional and dispositional properties in genetic and other lower-level explanations. Yet, they contend that all these seemingly higher-level concepts belong to the domain and vocabulary of molecular biology, broadly construed. Thus, the debate ultimately hinges not on the nature and depth of explanations, which are widely agreed upon, but on whether these explanations should be labelled as “molecular.” As a result, discussants talk past each other, making the dispute more terminological and less substantial than is typically assumed (Nathan 2012, under contract). With this in mind, let us return to psychology and neuroscience. Does the current psycho-neural interface vindicate or thwart reductionism? Do the brain sciences have the conceptual resources to describe all mental events in neural terms? And does this enhance their explanatory power? Well, much depends on how one characterizes the levels and vocabularies in question. Unsurprisingly, modest reductionists tend to presuppose a generous, ecumenical conception of “lower- level” descriptions. This includes functional, dispositional, and structural concepts, typically found at higher levels in the scientific hierarchy. In turn, sophisticated antireductionists, for the most part, agree on the importance of this explanatory apparatus. Yet, they are less liberal on what can be categorized as “lower-level,” “micro,” “neural,” or “molecular.” As in the biological case, discussants talk past each other and quibble over labels, making the debate semantic, as opposed to substantive. What moral should we draw from all of this? The take-home message is that philosophers of mind, psychology, and neuroscience should learn the hard lesson from their colleagues in biology. Important as they are, empirical discoveries concerning where and how cognitive functions are implemented in the brain are unlikely to solve any longstanding philosophical dispute over the mind-body problem. The reason is not the complexity of the human mind and brain—which, I emphasize once again, should not be questioned. The real problem is the nature of 12 The Mind-Body Problem 3.0 275 reduction which, contrary to common wisdom, turns out to be a murky construct. This lack of clarity becomes especially evident when one asks the question: does cognitive neuropsychology fit in better with reductionism or antireductionism? The answer is along the lines of both or neither. But does it even matter? Current psychology and neuroscience seem perfectly compatible with both stances, which is precisely what one could expect in the case of a merely verbal disagreement. In conclusion, following this stalemate, the philosophical debate over reduc- tionism has lost traction, as witnessed by the lack of resolution, coupled with the shortage of novel insight. Conceptual progress requires recognizing that the crux of the mind-body problem is independent of the muddled status of reduction. As Putnam noted long ago, the main issue is the question of explanatory autonomy. Since the 1970s, autonomy and reduction have been viewed as contradictory. Many scholars, and Putnam himself was no exception, view autonomy as the rejection of reduction and reduction as the denial of autonomy. This, we shall now see, is a consequential mistake. 12.6 The Mind-Body Problem 3.0 How should the mind-body problem be repackaged in the twenty-first century, given that its conceptual core—the bridge between the mental and the physical— is independent of both ontology and reduction, that is, versions 1.0 and 2.0? This section proposes a new framework for recasting Descartes’ dilemma, so as to reflect real, substantive disagreement across current scientific inquiry, while maintaining some continuity with its roots in early modern philosophy. To get started, let’s return to the relation between psychology and neuroscience. On the one hand, evidence concerning brain activity deepens, in several ways, higher-level psychological explanations. Discovering the inner workings of brain processes and networks sheds much light on why the mind works the way it does: its abilities, biases, and computational limitations. On the other hand, it seems equally undeniable that psychological descriptions and explanations enjoy an autonomy of sorts, in the sense that they can be established, corroborated, and explained without the aid of neuroscience or any other more fundamental discipline. Allow me to briefly elaborate. Consider some well-known psychological generalizations, such as the widespread tendency of subjects confronted with cases of moral decision making— such as the famous “trolley problems”—to follow consequentialist rules, unless doing so involves using other people directly as a means. Or take the “endowment effect,” which captures how the price that subjects are willing to accept to part with goods vastly exceeds the price they are willing to pay to acquire the same goods, violating core tenets of expected utility theory. In both cases, the generalizations themselves can be expressed, confirmed, refined, and explained, in purely psychological terms. To wit, the former generalization is typically accounted for by appealing to negative emotions clashing with consequentialist reasoning, 276 M. J. Nathan whereas the latter effect is often taken to depend on loss aversion. From this standpoint, learning more about the mechanisms which compute these cognitive patterns is no more necessary than physical details in Putnam’s square peg. Yet, the appropriate moral is not that fMRI cannot contribute to the study of higher cognition. Au contraire, so-called “reverse inferences” play a crucial role deepening these explanations by discriminating between competing psychological hypotheses (Del Pinal and Nathan 2013; Nathan and Del Pinal 2016). How can both points be maintained simultaneously? How can psychology be autonomous, while depending on the underlying neural substrate? How can we use neuroscience to advance the study of the mind, without threatening the indepen- dence of higher explanatory levels? These are the pressing questions pertaining to the mind-brain relation. Putnam had the right insight when he pointed the discussion towards autonomy. The crucial mistake was turning the issue of autonomy into a debate about reduction. The central philosophical question underlying current neuropsychological debates, I maintain, is how to pursue the scientific study of the mind in the age of neuroscience. This is what I call the mind-body problem 3.0. What makes 3.0 different from the previous versions 1.0 and 2.0? My goal is to sketch a constructive framework for recasting old questions in a new guise, thereby avoiding the thorny issues of ontology and reduction. The main hang-up can be posed in the form of a dilemma. On the one hand, neural details are crucial for understanding the structure, implementation, and behavior of psychological systems. This was the main insight of twentieth-century physicalism. At a bare minimum, brains set boundary conditions and constraints on what minds can or cannot do and why this is the case. But this being so, in what sense is the higher-level truly autonomous? On the other hand, if one begins by stressing the autonomy of psychology, it becomes hard to see why neural details should matter at all. There is a simple way out of this impasse. The problem with traditional formulations of materialism, including reductionist and antireductionist approaches alike, is presupposing, more or less explicitly, that higher-level explanations and their lower-level counterparts have the same explananda, the same objects of explanation. In essence, what discussants failed to recognize is that questions at different levels and with varying scope are, effectively, different questions. Some illustrations should help make the point clearer. First, let’s return to the square-peg-round-hole scenario. From a metaphysical standpoint, board and peg supervene in their atomic structure. This is true, albeit uncontroversial and inessential to the main point of contention. Descartes’ onto- logical concerns, mind-body 1.0, have long been put to rest, for good reason. The relevant issue is whether these micro-details enhance the power of the explanation of the system’s behavior. Putnam’s deep insight was recognizing that much depends on what we are trying to explain. If the explanandum is that the square peg will not pass through the round hole, then the micro-details can be effectively black-boxed. 12 The Mind-Body Problem 3.0 277 In contrast, if we are trying to capture why this is so, then looking at the physical structure of the system, down to its subatomic properties, becomes relevant. Now, apply this perspective to the psycho-neural interface. Are brain-level details relevant to the study of the mind? The short answer is that it depends. Some cognitive inquiries are framed in a way that makes them perfectly autonomous, in the sense that they can be confirmed, refined, and explained without the aid of physical, molecular, or even neural details. Cognitive hypotheses regarding the engagement of negative emotions in trolley problems or loss aversion in economic decision making do not require the aid of neuroimaging or other neuroscientific techniques. Still, this is not to deny that there are other, equally important questions to ask about how these higher-level functions are implemented, processed, or realized at more fundamental levels. It is here that neural details may become indispensable. The main point—stressed, in different ways, in both the scientific (Marr 1982) and the philosophical literature (Garfinkel 1981)—is that translating higher-level questions into lower-level ones, or vice versa, may yield different inquiries. Failure to recognize this has generated confusion. The misleading assumption, shared by reductionists and antireductionists alike, is that higher- and lower-level explanations are in competition. They are not. Borrowing a Kuhnian metaphor, explanations with different scope are typically incommensurable. Because of their different targets, any attempt to rank them in terms of explanatory power turns into an exercise in futility. In short, Putnam’s contribution was recognizing that higher-level explanations are epistemically “autonomous” from lower-level ones. His mistake, which wreaked much havoc in subsequent discussion in the philosophy of science and mind, was turning this into a vindication of antireductionism. Putnam, and many philosophers after him, identified autonomy with antireductionism. These concepts, I maintain, should not be conflated. Whereas reductionism and antireductionism disagree on whether more fundamental depictions should invariably be preferred over less- fundamental ones, both stances presuppose—indeed, require—convergence in the objects of explanations. Autonomy, in contrast, gains traction by rejecting this presupposition and embracing a form of epistemic incommensurability (Nathan, under contract). Before moving on, let me address how the present proposal fits in with two current debates in the philosophy of mind. First, readers may note some analogies between my presentation of the mind-body problem 3.0 and the “mechanistic turn” in the philosophy of neuroscience, which was born out of a reaction to the traditional reductionism vs. antireductionism divide (Bechtel and Richardson 2010). While the new wave of mechanistic philosophy is too sizable a movement to present, let alone assess, in a few statements, I should stress that, despite its popularity, it has not yet escaped the grip of the mind-body problem 2.0. Over the last few years, mechanistic accounts of explanation have been criticized based on the allegation that they are committed to the unpalatable tenet that adding any 278 M. J. Nathan kind of detail about a mechanism will improve an explanation (Batterman and Rice 2014; Chirimuuta 2014; Levy 2014). Neo-mechanists have responded by explicitly distancing themselves from this “more details are better” stance and replacing it with the thesis that that only relevant details improve an explanation (Baetu 2015; Boone and Piccinini 2016a; Craver and Kaplan 2018). My present perspective can be squared with the mechanistic joinder. First, not all details are relevant or helpful for every explanation. Second, which details matter will crucially depend on the explanandum at hand. Third, and finally, determining which details are relevant to an explanatory task is no simple task (Krickel and Kohar this volume). Yet, to avoid the grip of reduction, it is crucial to stress the incommensurability of explanations at different levels, a point seldom stressed explicitly, to the best of my knowledge. Second, as mentioned at the outset, the current discrepancy between traditional philosophy of mind and ongoing debates in the cognitive neurosciences has not gone unnoticed. For instance, Chemero and Silberstein (2008, p. 1) maintain that “The philosophy of mind is over.”8 Boone and Piccinini (2016b) take the argument one step forward by suggesting that cognitive science itself, as traditionally conceived, is currently in the process of being replaced by cognitive neuroscience. As a result, the old debate between reductionism and autonomy has faded into the background, replaced by a focus on multilevel mechanistic explanations.9 In contrast, I have tried to stress here the continuity between past and present debates. Nevertheless, the obvious differences between these proposals and the perspective defended here should not be overstated. Chemero and Silbertein’s holistic cognitive science, Boone and Piccinini’s multilevel mechanistic explanation and my attempted differentiation between autonomy and antireductionism all share a common assumption: in some form or another, philosophy still has an important role to play. The fundamental question of contemporary philosophy of mind is how to pursue the scientific study of the mind in the age of neuroscience. This, in essence, is the mind-body problem 3.0. 8 Chemero and Silberstein motivate their provocative claim as follows: “The two main debates in the philosophy of mind over the last few decades about the essence of mental states (they are physical, functional, phenomenal, etc.) and over mental context have run their course. Positions have hardened; objections are repeated; theoretical filigrees are attached. These relatively armchair discussions are being replaced by empirically oriented debates in philosophy of cognitive and neural sciences” (2008, p. 1). 9 “The scientific practices based on the two-level view (functional/cognitive /computational’ vs. neural/mechanistic/implementation) are being replaced by scientific practices based on the view that there are many levels of mechanistic organization. No one level has a monopoly on cognition proper. Instead, different levels are more or less cognitive depending on their specific properties. The different levels and the disciplines that study them are not autonomous from one another. Instead, the different disciplines contribute to the common enterprise of constructing multilevel mechanistic explanations of cognitive phenomena. In other words, there is no longer any meaningful distinction between cognitive psychology and the relevant portions of neuroscience— they are merging to form cognitive neuroscience” (Boone and Piccinini 2016b, p. 1510). 12 The Mind-Body Problem 3.0 279 12.7 Concluding Remarks Time to pull some strings together. I distinguished three variants of the mind- body problem. Version 1.0 reflects Descartes’ ontological quandary: what kind of substances are minds and how are they related to bodies? Version 2.0 tacitly underlies much twentieth century philosophy of mind: can mental states be reduced to brain states? Neither variant has been solved. Both have been dissolved, recast. Finally, I advanced a revamped “mind-body problem, version 3.0,” In a slogan, this is the question of how to pursue psychology, the modern science of the mind, in the age of neuroscience, the science of the brain. How should these two disciplines inform each other? Undoubtedly, many readers will remain unpersuaded by my contemporary reformulation of the mind-body problem. Property dualists maintain that the gap between physical and phenomenal properties is ontological, not merely epistemic (Chalmers 1996). Hylomorphists are concerned with the ontological irreducibility of macro-causes (Jaworski 2016; Koslicki 2018). These issues seem orthogonal to the problem of explaining the metaphysical relationship between higher-level properties and their micro-base. Version 3.0 implicitly strips the mind body problem of all its metaphysical underpinnings, including the insistence of some new-wave mechanists, in an ontic approach to explanation (Craver 2007). Does this unduly narrow its scope? My response is that it might be time to reshuffle the deck. Perhaps, issues which, prima facie, appear to be ontological in character could be fruitfully repackaged as questions of explanation and methodology. This proposal is supported by the observation that current scientific research can be made consistent with virtually all combinations of materialism, dualism, reductionism, and antireductionism. To illustrate, most contemporary scholars presuppose some variety of materialism— and yours truly is no exception. Yet, it has been pointed out that, in principle, current psychology could be reconciled with various forms of dualism (Chalmers 1996). Similarly, nothing substantial hinges on whether or not psychology is “reducible” to neuroscience. As noted, the answer depends on how exactly one conceives of reduction and how broadly the domain of lower-level theories is defined. Even contemporary uses of neuroimaging are compatible with various forms of antireductionism and ontological dualism (Del Pinal and Nathan 2013; Nathan and Del Pinal 2016). Paraphrasing Wittgenstein, these are matters of expression, not facts of the world. After centuries of discussion, it might be time to abandon old ontological questions and try out something new. My aim here transcended mere exposition and historical reconstruction. Both my pars destruens and pars construens advance critical analyses and suggestions for moving forward. Still, the succinct remarks contained in this article, by themselves, admittedly fall way short of a solution to the mind- body problem 3.0. Follow-ups await. Which inquiries should be prioritized? How does one determine whether a question is best explained at higher or lower levels? How much detail is relevant? Do explanatory standards cut across domains? Can we provide effective mappings of 280 M. J. Nathan scientific ontologies at different steps of the hierarchy? Providing answers requires a painstaking combination of empirical and conceptual work. In addition, we saw that there are various alternative proposals for addressing the relation between psychology and neuroscience along the lines suggested here. Should we opt for a holistic approach? A focus on multi-level mechanisms? A revamped version of autonomy? Something altogether different? While this is not the appropriate venue for weighting these options, it seems to me that reframing the mind-body problem in a way that avoids getting entangled in verbal disputes concerning ontology and reduction is a step in the right direction. I conclude by stressing two features of the present proposal. First, traditionally, the family of issues underlying the “mind-body problem” has been concerned with the exceptionality of human cognition. Cartesian dualists resist the identification of mind and matter by treating the former as ontologically distinct from anything else in the physical universe. Mid-twentieth-century reductionists, like Place and Smart, have advocated the treatment of psycho-neural reduction as a scientific hypothesis. Non-reductive physicalists, such as Nagel and Davidson, have responded by emphasizing features of the mental that make it unique. The same kind of tension can be found within the 3.0 version too. On the one hand, some philosophers might view the relation between psychology and neuroscience as a general issue in the philosophy of science. Just like there is a mind-body (qua psychology-neuroscience) problem, there is a biology-chemistry problem, an economics-sociology problem, etc. From a methodological perspective, all these interfaces are on a par. Others will disagree, for instance, by emphasizing features of the mental—a sui generis normativity, a “hard” problem of consciousness, or something along these lines— that make the mental special, or otherwise exceptional. Second, contrary to versions 1.0 and 2.0, mind-body 3.0 raises a problem that is central to contemporary scientific agendas. The outcome of the debate on whether and how psychology and neuroscience can mutually inform each other will likely determine how the study of mental and neural structures will be approached—and funded—over decades to come. It is crucial for philosophy to keep asking the right questions and focus on substantive conceptual and empirical issues that are central to core scientific practice, like it has done for much of its history. The disconcerting alternative is for philosophical analysis to become irrelevant and fade into oblivion. Acknowledgments The author is grateful to Bill Anderson, John Bickle, Fabrizio Calzavarini, Matteo Colombo, Guie Del Pinal, Carrie Figdor, Matteo Grasso, Philipp Haueis, Mika Smith, Marco Viola, and two reviewers for constructive comments on various versions of this essay, and to Stefano Mannone for designing the image. 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