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By Zach Blesi

1. Introduction

Contemporary metaphysics is marked by a revived interest in the notion of ground. This notion is commonly taken to be primitive and illuminated by way of example. A number of philosophers (call them grounding theorists) maintain that this notion is indispensable to the philosopher’s conceptual toolkit. However, while the recent literature on ground has focused heavily on its logic and connection to other notions in metaphysics, there has been significantly less detailed discussion concerning how the notion should be applied to particular philosophical issues. But that hasn’t stopped a number of grounding theorists from suggesting (typically in passing) that ground is exactly the notion needed to successfully develop physicalism in the philosophy of mind—the view that the mental is “nothing over and above” the physical.

Thus, Schaffer (2009) tells us that ground “is the notion the physicalist needs to explicate such plausible claims as ‘the fundamental properties and facts are physical and everything else obtains in virtue of them’” (364). Moreover, Rosen (2010) maintains that the debate between physicalists and dualists hinges on whether “the facts about phenomenal consciousness are grounded in, and hence necessitated by, the neurophysiological facts that underlie them” (118). And according to Dasgupta (2014):

One might try understanding the picture [of physicalism] as an identity thesis. . . . Or as a thesis of analysis. . . . Or as a supervenience thesis. . . . But the recent interest in ground stems largely from the idea that these formulations do not fully capture the picture, and that we should instead understand it in terms of ground. (558)

It is far from clear that the notion of ground is indispensable to stating the minimal commitments of physicalism (cf. Melnyk 2016). Nonetheless, this enthusiasm for the significance of ground to physicalism is not without warrant. For there are reasons to think that the notion of ground can be used to develop a version of physicalism (call it Grounding Physicalism) that has significant advantages over the traditional options. The appeal of Grounding Physicalism is that it promises to occupy a middle position between reductive and non-reductive versions of physicalism. On the face of it, Grounding Physicalism is less demanding than reductive versions of physicalism, because it does not require that mental phenomena be identified with (or defined in terms of) physical or functional phenomena. On the other hand, unlike some versions of non-reductive physicalism, it can adequately capture the idea that the mental depends on and is explained by the physical.

The primary aim of this paper is to undermine the enthusiasm for Grounding Physicalism by putting a new spin on a common objection to physicalism: that it leaves an “explanatory gap” (Levine 1983, 1993). This objection has been heavily discussed in the philosophy of mind. However, a number of these discussions proceed by simply assuming that physicalism is fundamentally an identity thesis. By contrast, I will take Grounding Physicalism as my target and argue that it leaves an explanatory gap—moreover, one which cannot be addressed in the usual way.

I believe it is possible to close this gap, but it comes at a price: either identify mental phenomena with physical phenomena or provide “real definitions” of mental phenomena in physicalistically acceptable terms. My central claim is that this presents the Grounding Physicalist with a dilemma: either leave an explanatory gap or abandon the considerations that motivated Grounding Physicalism in the first place.

The paper proceeds as follows. In §2, I state Grounding Physicalism explicitly. In §3, I argue that the grounding facts and patterns involving consciousness are to be explained (if at all) by essentialist truths about consciousness, even if those essentialist truths do not ground any grounding facts or patterns. In §4, I provide reasons for thinking that Grounding Physicalism leaves an explanatory gap. Finally, in §5, I argue that a popular response to the explanatory gap problem is unavailable to the Grounding Physicalist, and I establish my dilemma. The upshot of the dialectic is a dose of pessimism for the usefulness of the notion of ground to the mind–body problem.

2. Grounding Physicalism

To make Grounding Physicalism explicit, I need to introduce some important provisos. First, I will take ground to be an explanatory notion (cf. Dasgupta 2014, 558). Thus, for any p and q, if [p] grounds [q], then [p] metaphysically explains [q]. Second, I will assume that grounding is a relation between facts, and I will follow Rosen (2010) in understanding facts to be “structured entities built up from worldly items—objects, relations, connectives, quantifiers, etc.—in roughly the sense in which sentences are built up from words” (114). (For the sake of brevity, I will occasionally speak as if properties, states, and events are also relata of the grounding relation; on such occasions, the reader should understand my claims in terms of facts involving those entities.) Finally, I will take the grounding relation to be transitive, asymmetric, irreflexive, and necessitating. To say that grounding is necessitating is to say that for any collection of facts p 1 , p 2 , … ∈ ∆ and any fact q, if p 1 , p 2 , … ground q, then it is metaphysically necessary that if ∧∆ is the case, then q is the case.

With those provisos in place, Grounding Physicalism in the philosophy of mind is the following thesis:

Grounding Physicalism: The facts about consciousness are fully grounded in the physical facts.

By “fully grounded,” I mean that there is some chain of ground leading up from the physical facts to the facts about consciousness such that there is no fuller account of why the facts about consciousness obtain. As a result, the physical facts might fully mediately ground the facts about consciousness by grounding various intermediate facts (such as chemical, biological, or neurological facts).

Consequently, Grounding Physicalism entails that for any fact about consciousness p, there is some non-empty collection of physical facts ∆ such that:

Necessitation: □(∧∆ ⊃ p).

However, Grounding Physicalism does not entail necessitation in the other direction (cf. deRosset 2013, 6). It is for this reason that Grounding Physicalism, unlike the mind–brain type-identity theory, is consistent with the multiple realizability of mental states.

Grounding Physicalism also has two big advantages over supervenience-based non-reductive physicalism (SBNP for short). The latter has been repudiated for incurring a commitment to brute modal correlations of simple phenomenal properties with complex physical properties—“modal danglers” that count against the view for the same reason that brute psychophysical laws count against property dualism (Smart 1959). As Kim (1989) puts it, “If the global supervenience of the mental on the physical were to be proposed as an unexplainable fact that we must accept on faith, I doubt that we need to take the proposal seriously” (42). For related reasons, Horgan (1993) argues that physicalism requires superdupervenience: “ontological supervenience that is robustly explainable in a materialistically acceptable way” (577). Grounding Physicalism, by contrast, entails the global supervenience of the mental on the physical (Schaffer 2017, 20). For if Grounding Physicalism is true, then the physical facts necessitate the mental facts. And if the physical facts necessitate the mental facts, then any possible world that is a minimal physical duplicate of the actual world will be a mental duplicate of the actual world. So, if Grounding Physicalism is true, the supervenience of the mental on the physical is to be explained by the nature of ground.

SBNP has also been criticized for being consistent with emergentism (see Crane 2010; Horgan 1993; Wilson 2005). Grounding Physicalism, however, appears to rule out emergentism. For since ground is an explanatory notion, the mental facts are ultimately explained by the physical facts. Moreover, according to a popular account of fundamentality, if consciousness is metaphysically grounded, then consciousness is non-fundamental (cf. Dasgupta 2014, 563). If emergentism is the view that consciousness is a fundamental, physically unexplainable phenomenon in the sense that it is not fully grounded in the physical, then Grounding Physicalism and emergentism are inconsistent. Thus, Grounding Physicalism has more resources than SBNP to secure nothing-over-and-aboveness.

Finally, Grounding Physicalism is immune to the charge (often raised against reductive physicalism) that it leaves out the most important part of consciousness. That is because Grounding Physicalism does not entail that phenomenal properties are reducible to physical or functional properties. It is consistent with Grounding Physicalism, for example, that the essence of pain is exhausted by its “what-it’s-likeness.” That is why I earlier suggested that Grounding Physicalism promises to occupy a middle position between traditional versions of reductive and nonreductive physicalism.

This completes my brief sketch of Grounding Physicalism. I now turn to the main aim of the paper: to establish my dilemma for Grounding Physicalism.

3. Grounding and Essence

3.1 The grounding facts

As I understand it, Grounding Physicalism is the thesis that the facts about consciousness are grounded in the physical facts. This commits the Grounding Physicalist to the existence of grounding facts: facts about what grounds what. To see why this is problematic, it is important to note that a number of grounding theorists presuppose the following claims about fundamentality:

Fact Fundamentality: For all facts p, p is fundamental iff p is ungrounded.

Entity Fundamentality: For any entity x (object, property, etc.), x is fundamental iff x figures in an ungrounded fact.

Call the conjunction of Fact and Entity Fundamentality the Grounding Account of Fundamentality. It is endorsed by Bennett (2011), Dasgupta (2014), deRosset (2013), and Litland (forthcoming). Some grounding theorists even motivate a commitment to the notion of ground by maintaining that fundamentality can be defined in terms of it. Indeed, Wilson (2014) holds that without a primitive notion of ground, we must take the notion of fundamentality itself to be primitive.

Now, suppose the fact that Jones is in pain ([P]) is grounded in the fact that Jones’ c-fibers are firing ([Q]). That is:

(1) [P] ← [Q].

We can now ask: Is (1) grounded or ungrounded?

There are two major problems with answering that (1) is ungrounded. First, if (1) is ungrounded, then the fact that (1) is ungrounded cries out for explanation. After all, the only relevant difference between (1) and, say, [P] is that (1) has the grounding relation as a constituent. So why, according to the Grounding Physicalist, should [P] be grounded but (1) not? It might be responded that the fact that the basic laws of physics are ungrounded likewise cries out for explanation and has none. The difference is that, plausibly, physicalism entails that there is no explanation of why the laws of physics are ungrounded—it is just an empirical postulate of physics that they are.

Second, given the Grounding Account of Fundamentality, if (1) is ungrounded, then since c-fiber firing and pain are constituents of (1), Grounding Physicalism entails that c-fiber firing and pain are fundamental—even worse, that Jones’ c-fiber firing and Jones’ pain are fundamental. On this option, Grounding Physicalism countenances a proliferation of fundamental facts and entities. This is an intolerable result. For even if there is reason to think that phenomenal and neural types are fundamental, there is no reason to think that their particular instances are fundamental.

On the other hand, if (1) is grounded, then there must be some fact [P′] that grounds it. But now the question arises as to whether [P′] is physical or not. If [P′] is apt to be explained—which is highly plausible to assume—then there is considerable pressure on the Grounding Physicalist to deny that [P′] is nonphysical. In the first place, Grounding Physicalism is typically motivated by a global grounding thesis: that every fact about (concrete) reality that is apt to be explained is either physical or fully grounded in some collection of physical facts. But if [P′] is nonphysical, then the global thesis is false, and one might then wonder why Grounding Physicalism should be accepted. Could it be that [P′] is nonphysical but grounded in some physical fact [P]? Grant this possibility for the sake argument. Given the transitivity of ground, there is still the fact that [P] grounds (1). We can now ask whether that fact is grounded or not. If it isn’t, then the earlier problems simply arise at a higher level.

It seems, then, that the Grounding Physicalist is forced to maintain that (1) is grounded in something physical after all. This is the line that Bennett (2011) and deRosset (2013) adopt. They maintain:

Collapse: For all p and q, if [p] ← [q], then [[p] ← [q]] ← [q].

This principle entails that (1) itself is grounded in the fact that Jones’ c-fibers are firing.

But how plausible is Collapse? Dasgupta (2014) provides two important reasons to reject it. First, Collapse implies that facts with intuitively distinct grounds are grounded in the very same fact. Consider disjunction. The fact that [P] grounds [P ∨ Q]. But what grounds the fact that [P] grounds [P ∨ Q]? According to Collapse, the fact that [P] grounds [P ∨ Q] is itself grounded in [P]. However, since the fact that [P] also grounds [¬¬P], Collapse implies that both grounding facts are grounded in the very same fact: namely, [P]. But that seems wrong: the fact that [P] grounds [P ∨ Q] has something to do with the nature of disjunction, whereas the fact that [P] grounds [¬¬P] has something to do with the nature of double negation.

Second, Collapse fails to provide satisfying explanations of facts like (1). For example, the existence of Socrates grounds the existence of {Socrates}. Now suppose someone asks why the existence of Socrates grounds the existence of {Socrates}, and Collapse delivers the following answer: “Because Socrates exists.” This is not a satisfying explanation. We understood that Socrates exists, but what we asked is why Socrates’ existence grounds {Socrates}. If we already found this mysterious, to be told that Socrates exists would not make it any more intelligible. Dasgupta then asks us to compare Collapse’s explanation with a competing one: “Because it is essential to being a set that for any x, if x exists, then the existence of x grounds the existence of {x}.” This explanation is clearly superior.

The previous two problems for Collapse suggest that essentialist truths play some important role in explaining the grounding facts. I will follow Fine (1994, 1995a) in taking essentialist truths to have the logical form of □xP, i.e., it lies in the essence (or nature) of x that P. I will also limit the class of essentialist truths to those that specify the constitutive essence of a thing (Fine 1995b). So, as I understand it, the essentialist truths with respect to a given thing specify (in part) what it is to be that thing in its most core respects.

Dasgupta (2014) and Rosen (2010) are sympathetic to:

Brute Essentialism: For any p and q, if [p] ← [q], then [[p] ← [q]] is grounded in [q] together with an essentialist truth about some constituent of [[p] ← [q]].

Brute Essentialism suggests that the fact that [P ∨ Q] ← [P] is itself grounded in [P] together with the fact that it lies in the nature of disjunction that for all propositions p and q, if p is true, then [p] grounds [p ∨ q]. Similarly, Dasgupta suggests, (1) is partly grounded in an essentialist truth about pain. And if that is right, then, as I will argue in §4 below, Grounding Physicalism is already in big trouble.

However, there are two important reasons to resist Brute Essentialism. First, in response to Dasgupta’s first objection to Collapse, the Grounding Physicalist might endorse Collapse while maintaining that facts can be grounded in different ways. For example, it might be said that the fact that [P ∨ Q] ← [P] and the fact that [¬¬P] ← [P] are both fully grounded in [P]. The difference is that the way of ground for disjunction corresponds to disjunction introduction, whereas the way of ground for negation corresponds to double negation introduction (see Litland forthcoming, §9).

Second, analogies with logic and causation suggest that Brute Essentialism is founded on a mistake (cf. Bader forthcoming, 12). If the premises P and P ⊃ Q are true, then the conclusion Q logically follows by the inference rule modus ponens. But modus ponens is not a premise in the argument; rather, the inference rule somehow “connects” the premises to the conclusion. Likewise, while a causal law L might somehow “govern” the causal relations between A-like events and B-like events, L does not cause any of those events. Moving from the claim that essentialist truths play some role in explaining the grounding facts to the claim that essentialist truths partly ground the grounding facts is like moving from the claim that inference rules connect premises to conclusion (or that causal laws govern causal relations) to the claim that those inference rules are premises in arguments (or that causal laws cause events).

In short, Dasgupta simply assumes that if essentialist truths provide for explanations of the grounding facts, then those explanations must be grounding explanations. In response, the Grounding Physicalist might endorse Collapse while maintaining that essentialist truths explain the grounding facts in some other sense. For example, it might be argued that the fact that [P ∨ Q] ← [P] is fully grounded in [P], but (looking to) the essence of disjunction somehow makes that grounding connection intelligible. Still, this discussion has been instructive. It suggests that there is an explanatory connection between ground and essence.

It is worth pausing to carefully appreciate where we are in the dialectic. The Grounding Physicalist is committed to the existence of grounding facts. Even if Collapse is true, these facts clearly cry out for explanation in some sense, and essentialist truths play a crucial role in securing such explanations. The Grounding Physicalist is not yet in trouble. Perhaps the fact that c-fiber firing grounds pain is to be unproblematically explained by the essence of something other than just pain. I now turn to showing why this suggestion won’t work by looking at a related grounding phenomenon.

3.2 The grounding patterns

Suppose that the fact that Jones is in pain is grounded in the fact that his c-fibers are firing at rate 1.5 Hz. It is plausible to think that if another person’s c-fibers were firing at rate 1.5 Hz, then that person, too, would experience pain. And so on for any other (possible) individual. This suggests that there exists a grounding pattern involving c-fiber firing and pain. Drawing from Fine (2012), we can express this idea precisely. Suppose that [Q] grounds [P]. [P] and [Q] will have as constituents certain existing items a 1 , a 2 , … and b 1 , b 2 , …, respectively. Using [P(a 1 , a 2 , …)] and [Q(b 1 , b 2 , …)] to represent this, we can generalize away from the particular grounding connection to:

Grounding Pattern: Necessarily, for any x 1 , x 2 , … and y 1 , y 2 , …, if ϕ(x 1 , x 2 , …), ψ(y 1 , y 2 , …), and Q(y 1 , y 2 , …) is the case, then [P(x 1 , x 2 , …)] ← [Q(y 1 , y 2 , …)],

where ϕ(x 1 , x 2 , …) and ψ(y 1 , y 2 , …) are conditions that in fact hold of a 1 , a 2 , … and b 1 , b 2 , …, respectively. If this grounding pattern holds, then the fact that [Q] grounds [P] logically follows from it.

To put the idea loosely, any time some physical event grounds an experience of pain, say, that event has some property such that, necessarily, every event that has that property grounds the experience of pain. The principle may not be completely obvious, but it is easy to multiply examples in which grounding connections entail grounding patterns. Thus, if the ball is scarlet, then its being scarlet grounds its being red. But that’s true of any ball, and if the ball had been crimson instead, its being crimson would have also grounded its being red. The determinate properties of being scarlet and being crimson share something in common with all determinate shades of red: they ground the instantiation of red whenever they are instantiated.

The central question to be explored here is what explains the grounding patterns involving consciousness. One possibility is to say that these patterns are grounded in and thus explained by their instances. The advocate of Collapse might then argue that the grounding patterns do not pose some further problem. For if the grounding patterns are to be explained by the particular grounding facts, then the real mystery is why each particular grounding connection holds. If only we could explain every grounding fact, we would have a complete explanation of the grounding patterns. No further mystery would remain.

There are two problems with this proposal. In the first place, each of these universal generalizations falls within the scope of a necessity operator, and while there is no consensus on what grounds necessitated truths, no one says that necessitated universal generalizations are even partially grounded in their instances. So, it’s simply false that we can explain the grounding patterns involving consciousness by explaining, in piecemeal fashion, why each particular grounding connection holds. But for the sake of argument, let’s set the necessity operators aside. It’s true that if each universal generalization is grounded in its instances, then the grounding patterns are metaphysically explained by their instances. But that isn’t the sort of explanation we were looking for. As Dasgupta (2014) puts it, “We want to know why all those instances turned out alike—just repeating the instances is no answer” (570). Just as the facts about what grounds what cry out for explanation (even if Collapse is true), so also the grounding patterns cry out for explanation (even if they are grounded in their instances).

Could it be that the grounding patterns are to be explained by the nature of some property that the grounds all share? In general, this isn’t the case. It’s true that for any x, if x exists, then its existence grounds {x}. From numbers to people, this pattern holds. But there is nothing common to numbers and people that could explain the grounding pattern involving sets and their members. To put the point more abstractly, it doesn’t lie in the nature of existence or entityhood that the existence of entities grounds the existence of their sets.

In addition, according to Rosen (2010), if the grounding pattern involving pain and c-fiber firing is to be explained by the nature of c-fiber firing, then “the analgesic neuroscientist who knew everything about the detailed physiology of c-fibers and their role in the functional economy of the organism but who knew nothing about pain would have an incomplete understanding of what it is for a c-fiber to fire” (133). But that seems implausible: “[I]t is hard to see why [the neuroscientist’s] understanding of the essence or definition of this particular neurological kind should be defective” (133).

Perhaps the grounding patterns are to be explained by the nature of some property the grounds all share together with the nature of ground (cf. Fine 2012, 77). On this suggestion, there exists an extremely complicated truth of the form □ground (P ∧ Q ∧ …), which somehow mentions each and every grounding pattern involving consciousness. There are three major problems with this view. First, it just doest doesn’t seem to be in the nature of ground that the grounding patterns hold: if one is ignorant of a single grounding pattern, one is not thereby ignorant of what ground is. That is why the colorblind metaphysician might fully grasp the essence of ground despite knowing nothing of the grounding patterns involving color. Second, on this view, if at least one grounding pattern is knowable only a posteriori (as seems highly plausible), then it is impossible to grasp the full essence of ground a priori. This suggests that the epistemology of ground is perversely unlike the epistemology of every other notion in metaphysics. Finally, it is difficult to understand why the essence of ground should mention only the grounding patterns that in fact hold. It’s true that, necessarily, for any x, if x is scarlet, then x’s being scarlet grounds x’s being red, while it’s false that, necessarily, for any x, if x is scarlet, then x’s being scarlet grounds x’s being blue. Why is that? On the current proposal, the best we can say is that it lies in the nature of ground that the first proposition be true, while it does not lie in the nature of ground that the second proposition be true. But that seems more ad hoc than informative.

My claim is that the grounding patterns involving consciousness are to be explained (if at all) by the nature of consciousness. This follows from a general picture of ground: that grounding patterns are always to be explained by the natures of grounded items. As Fine (2012) puts it, “It is the fact to be grounded that ‘points’ to its grounds and not the grounds that point to what they may ground” (76). A handful of examples lend to the plausibility of this view. Why does the existence of a plurality of objects ground the existence of the set of those objects? Because it lies in the nature of a set that if its members exist, then those members ground the existence of the set of those members. Why does the instantiation of a determinate shade of red ground the instantiation of red? Because it lies in the nature of red that if some object is a determinate shade of red, then its being that shade of red grounds its being red. Indeed, it is hard to find examples in which the essence of a genuinely grounded fact doesn’t point to its grounds (cf. Greenberg 2001, 173). However, my arguments do not depend on the general picture. It is enough for my purposes that the picture holds with respect to the grounding patterns involving consciousness, even if it fails to hold across the board.

It might be objected that the physicalist should maintain that the grounding patterns are to be explained by “metaphysical laws,” not essentialist truths. After all, there is some plausibility to the idea that the grounding pattern involving red and the determinate shades of red is to be explained by a determinable–determinate law; the grounding pattern involving sets and their members by a set-formation law; and the remaining grounding patterns by various other “construction–operation laws” (Wilsch 2015). So why not posit some psychophysical metaphysical laws as well? In response, unlike a determinable–determinate law or a set-formation law, it is extremely unlikely that there is a single metaphysical law that subsumes and fully explains every grounding pattern involving consciousness. At best, there will be a whole range of metaphysical laws to reflect a wide range of phenomenal experiences. But these laws won’t be the sorts of things we can know a priori, nor is it clear that we could discover them a posteriori. Perhaps these challenges can be met. Ultimately, the decisive reason to reject this proposal is that it is dialectally inimical to Grounding Physicalism. It forces the Grounding Physicalist to substitute brute metaphysical laws for brute supervenience relations. Much like the psychophysical laws of property dualism, these “metaphysical danglers” cry out for explanation and yet have none. The essentialist picture is not obviously subject to this same criticism. Essentialist truths specify what it is to be something. They are “autonomous” in that they are not apt to be explained in the first place (Dasgupta 2014, 2016).

If the essentialist picture is right—and the shortcomings of the alternatives suggest that it is—then the grounding patterns involving pain are to be explained by the nature of pain. The arguments for this picture also show why the grounding facts cause trouble for Grounding Physicalism. For the essence of c-fiber firing no more explains the particular instances of the grounding pattern involving c-fiber firing and pain than it explains the grounding pattern itself. This is the first step to establishing my dilemma for Grounding Physicalism.

It is of crucial importance to note that these explanations need not be grounding ones. That is, essentialist truths might explain the grounding facts and patterns without themselves grounding them. As a result, even if the Grounding Physicalist endorses Collapse, there still significant pressure on the Grounding Physicalist to accept the claim that the grounding facts and patterns involving consciousness are to be explained by the essence of consciousness itself.

4. The Explanatory Gap

For convenience, I will refer to the facts about what grounds what and the grounding patterns as grounding phenomena. In the previous section, I argued that the grounding phenomena involving consciousness are to be explained by truths about the nature of consciousness. The problem, to be explored in this section, is that there just doesn’t seem to be anything about the nature of consciousness that could explain the grounding phenomena involving it. This requires first saying something about the notion of explanation. A detailed assessment falls outside the scope of this paper, but I take it that there is a perfectly respectable sense of “explanation” in which one phenomenon is (or can be) made intelligible in terms of another (cf. Levine 1983, 358). Following Jenkins (2008), we might also say that “explanations are things that can provide answers to why-questions” (71). Some why-questions pertain to grounding phenomena, and they are to be answered by the essences of things. My claim is that the essence of consciousness does not make the grounding phenomena involving consciousness intelligible, nor does it answer the crucial why-questions pertaining to those grounding phenomena. As a result, Grounding Physicalism fails to explain those phenomena—that’s what I mean when I say that Grounding Physicalism leaves an explanatory gap.

This is not to say that experiences are unexplained by their putative grounds. After all, since ground is an explanatory notion, if a given conscious experience really is grounded in a physical event, then that conscious experience is metaphysically explained by that physical event. Instead, the claim is that there is no explanation as to why physical events give rise to certain conscious experiences rather than others. For putative grounding patterns like the following cry out for explanation:

(2) Necessarily, for any x, if x’s c-fibers are firing, then the fact that x’s c-fibers are firing grounds the fact that x is in pain.

Yet, when we look to the essence of pain, I submit, we learn nothing to rule out the possibility of there being an x such that x’s c-fiber firing grounds x’s experiencing pleasure. So, Grounding Physicalism leaves an explanatory gap in one traditional sense of failing to explain why, in general, having one’s c-fibers fire should feel the way that it does rather than some other way or no way at all (Levine 1983, 358).

But why accept the claim that the essence of pain fails to explain the putative grounding patterns involving it? While it would take a paper-length treatment to motivate this claim in any detail, it is enough for my purposes to simply establish its plausibility. So, I will briefly discuss two reasons for accepting it.

First, suppose that c-fiber firing is one possible ground for pain. In order to explain the grounding patterns involving pain and c-fiber firing, the Grounding Physicalist must maintain that:

(3) □pain ∀x(x’s c-fibers are firing ⊃ ([x is in pain]←[x’s c-fibers are firing])).

It may be that (3) specifies only the mediate essence of pain (Fine 1995a). For intuitively, the immediate essence of a thing includes only what has a direct bearing on that thing. For example, while it may be of the immediate essence of {Socrates} to contain Socrates, and of the immediate essence of Socrates that he originate from the particular sperm–egg pair from which he actually originated, it is only of the mediate essence of {Socrates} to contain an individual that originated from a particular sperm–egg pair. So it may be that we can only arrive at (3) by chaining the immediate essences of various things together. I will take no stand on what those things might be. The point remains: if c-fiber firing is one possible ground for pain, then the Grounding Physicalist is committed to the truth of (3).

The problem with (3) is that zombies—systems that are physically and functionally indiscernible from ordinary human beings but that are completely unconscious— are conceivable. The claim that conceivability entails metaphysical possibility is controversial, so zombies might be metaphysically impossible. But the conceivability of zombies at least provides defeasible evidence for thinking that propositions like (3) are false (cf. Pautz 2014, 170).

Second, many philosophers are sympathetic to a thesis of revelation with respect to phenomenal properties like pain. According to the thesis of revelation, in experiencing pain, for example, one is an epistemic position to know all the essentialist truths about pain. The problem for Grounding Physicalism is that, if revelation is correct, one should be in an epistemic position to know that (3) is true on the basis of painful experiences alone. But one is in no such position. If you are skeptical, then the next time you stub your bare toe on a piece of furniture, just pay extra attention to the unpleasant experience you undergo and ask yourself how in the world you are supposed to get from that to anything like (3).

The Grounding Physicalist might try to produce some defeater for the evidence we get from conceivability and revelation. A priori type-identity theorists, for example, insist that zombies only seem to be conceivable and so there is no real evidence to be defeated. But how will the Grounding Physicalist respond? After all, the Grounding Physicalist denies that phenomenal properties are identical to physical/functional properties. My provisional conclusion is that Grounding Physicalism leaves an explanatory gap. This is the second step to establishing my dilemma for Grounding Physicalism.

5. Closing the Gap

The popular response to the claim that physicalism leaves an explanatory gap is to argue that if physicalism is true, nothing has been left unexplained and, therefore, there is no explanatory gap. For example, Tye (1999) tells us:

Take the referent of the term ‘Q’ and the referent of the term ‘this feeling’—conceive of those referents as you will—why is the former the same as the latter? If this is how the question is understood, then there is no significant question here for the physicalist. Only one state exists, conceived of in two ways, and that state must be self-identical. On this interpretation, then, there is no need for an answer and no explanatory gap. (717)

Similarly, according to Papineau (2002):

A mind-brain identity simply says of something that it is itself. . . . I say that once you really accept that pain, say, really is some material M, then you will see that this requires no more explanation than does Mark Twain = Samuel Clemens. Identities need no explaining. (150; emphasis added)

Rather, the so-called “explanatory gap” is a “cognitive illusion” (Tye’s phrase), the remnant of an “intuition of distinctness” (Papineau’s phrase). Balog (2012a,b), Block and Stalnaker (1999), and Block (2007) more or less follow suit.

Notice, however, that this response to the explanatory gap problem presupposes a type-identity theory. The central claim is that it makes no sense to explain why one thing is identical to itself. By contrast, the Grounding Physicalist denies that phenomenal properties are identical to physical or functional properties. The problem that the Grounding Physicalist confronts is that it does make sense to explain grounding facts and patterns. Grounding phenomena, unlike identities, are apt to be explained. Therefore, if Grounding Physicalism is true, it leaves a genuinely explanatory gap, not a mere cognitive illusion.

The Grounding Physicalist’s only hope of closing the explanatory gap is to say something about the nature of consciousness that accounts for the grounding phenomena involving it. This does not automatically commit the Grounding Physicalist to giving a reductive account of consciousness. There are a number of non-reductive positions that arguably incur essentialist commitments. The nonreductionist about determinable colors might deny that there is a full real definition of red in terms of the determinate shades of red but concede that it is essential to red that for any x, if x is scarlet, then the fact that x is scarlet grounds the fact that x is red. It is open to the Grounding Physicalist to likewise claim that the essence of pain lists out each of its grounds, even though there is no biconditional analysis of pain in terms of those grounds. There is still a big difference between these positions. The non-reductionist about color is not committed to giving even a partial analysis of red in non-color terms. By contrast, the Grounding Physicalist must say that the essence of pain mentions properties of a radically different kind. We might call such a position partially reductive.

However, there is a good case to be made that the Grounding Physicalist is ultimately committed to providing a full reductive account of consciousness. Consider the paradigm cases in which we have reason to posit essentialist truths with respect to a given phenomenon without giving a full reductive account of that phenomenon. These involve determinables and determinates, sets and their members, disjunctions and their disjuncts, and so on. In these cases, we have an a priori basis for positing essentialist truths. This is not a good model for the Grounding Physicalist. For even if it is essential to pain that for any x, if x’s c-fibers are firing, then the fact that x’s c-fibers are firing grounds the fact that x is in pain, that does not seem to be the sort of thing we are in a position to know a priori. In fact, that looks like a statement of the mediate essence of pain, which is ultimately to be derived from the yet to be discovered immediate essence of pain.

The Grounding Physicalist should instead appeal to cases of scientific discovery for a model. Consider the case of jade. Chemistry has revealed something important about the nature of jade. We might describe this discovery in one of two ways. First, we might hold that jade has a functional nature: the property of being jade just is the second-order property of having some property or other that plays the jade role. This puts us in a position to know that it is mediately essential to jade that for any x, if x is composed of jadeite, then the fact that x is composed of jadeite grounds the fact that x is jade. Second, we might hold that jade has a disjunctive nature: the property of being jade just is the property of being either jadeite or nephrite. On this proposal, it is immediately essential to jade that for any x, x is jade if and only if x is composed of either jadeite or nephrite. Either way, we have a reductive account of jade. The other paradigm cases of scientific discovery fit this pattern.

This suggests that the Grounding Physicalist’s only hope of closing the explanatory gap through empirical investigation is to identify phenomenal properties with physical/functional properties or to provide full real definitions of phenomenal properties in physicalistically acceptable terms. However, if the Grounding Physicalist can pull this off, Grounding Physicalism will no longer be a nonreductive view. Yet, one of the primary motivations for Grounding Physicalism is that it promises to occupy a middle position between reductive and non-reductive versions of physicalism. Therefore, if my overall argument succeeds, Grounding Physicalism faces a dilemma:

Dilemma: Either Grounding Physicalism leaves a genuinely explanatory gap, or the Grounding Physicalist must provide a reductive account of consciousness, thereby giving up one of the central motivations for the view.

So, what about just accepting the second horn of the dilemma? The problem with that response is that a number of philosophers—both physicalists and antiphysicalists alike—have concluded that no reductive account of consciousness will ever succeed. It is for this reason that Grounding Physicalism, as I have understood it, is such an enticing option. The problem (to continue the point) is that to avoid leaving an explanatory gap, Grounding Physicalism ultimately requires the same philosophical tools that philosophers of mind have been using for decades (e.g., reduction). (This conclusion should be friendly to Wilson (2014, 2016).)

The alternative is to accept that Grounding Physicalism leaves an explanatory gap. Some philosophers, I imagine, are willing to pay this price to avoid the problems of rival accounts. But many will take the existence of the explanatory gap to show that there is an ontological gap between the physical and the mental. These philosophers, like Fine (2012), might give up the claim that consciousness is metaphysically grounded in the physical and instead defend the weaker claim that it is naturally grounded in the physical. These philosophers will find in turn that they have arrived at a version of dualism. It remains to be seen which cost is greater.

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Zach Blesi is a fourth-year PhD student at the University of Texas at Austin. He works primarily on issues at the intersection of the philosophy of mind and metaphysics.