Not Your Grandma’s Phenomenal Idealism

[Author’s note: Now that I’m finally putting my stuff online, I have a backlog to get through. This is a term paper I wrote three years ago. As such, it reflects my views then, not necessarily now, and also it is optimized for being a term paper in a philosophy class rather than an academic paper for an interdisciplinary or AI-safety audience. Sections I and II sketch a theory, Lewisian Phenomenal Idealism, which is relevant to consciousness, embedded agency, and the building-phenomenal-bridges problem. The remaining sections defend it against objections that you may not find plausible anyway, so feel free to ignore them. Here is the gdoc version if you want to give detailed comments or see the footnotes.]

Not Your Grandma’s Phenomenal Idealism

[T]he structure of this physical world consistently moved farther and farther away from the world of sense and lost its former anthropomorphic character …. Thus the physical world has become progressively more and more abstract; purely formal mathematical operations play a growing part.  —Max Planck

When you see something that is technically sweet, you go ahead and do it and argue about what to do about it only after you’ve had your technical success. That is the way it was with the atomic bomb.  —Robert Oppenheimer

It started as just a weird theory invented to serve as a counterexample, but now I’m halfway convinced that it’s true.  —Daniel Kokotajlo

In the second chapter of his forthcoming book, “Idealism and the limits of conceptual representation,” Thomas Hofweber argues that phenomenal idealism is not worth taking seriously, on the grounds that it conflicts with what we know to be true—for example, we know that there were planets and rocks before there were any minds. (p12) I think this is too harsh: While we may have good reasons to reject phenomenal idealism, geology isn’t one of them. I think that phenomenal idealism should be taken as seriously as any other weird philosophical doctrine: The arguments for and against it must all be considered and balanced against one another. I don’t think that empirical considerations, like the deliverances of geology, have any significant bearing on the matter. The goal of this paper is to explain why.

I will begin by locating phenomenal idealism on a spectrum of other views, including reductive physicalism, which I take to be the main alternative. I will then present my own version of phenomenal idealism—“Lewisian Phenomenal Idealism,” or LPI for short—which I argue neatly avoids most of the standard objections to phenomenal idealism. It’s a proof of concept, so to speak, that phenomenal idealism is worth taking seriously. Finally, and most importantly, I’ll defend LPI against the strongest objection to it, the objection that it illegitimately reassigns the referents of the terms in the sentences we know to be true, and hence conflicts with what we know to be true.

I: Lewisian Phenomenal Idealism

Hofweber’s definition of phenomenal idealism is as follows:

The immediate objects of perception are thus phenomena, mental things which we are directly aware of. So far this is just a view about perception. But what turns it into idealism is to add that the objects are or are constructed from these phenomena, somehow. The material world is not a mind-independent world that we see indirectly, via the phenomena, nor is it represented by the phenomena. Instead, the phenomenal world is the material world. The phenomena do not represent the objects, but constitute them. (p20)

Though Hofweber’s definition above commits phenomenal idealism to two theses—one about perception, and another about ontology—I will only focus on the latter, since it generally taken to be more central. The phenomenal idealist position that I aim to defend is a version of the “Reductive Phenomenal Idealism” on the following spectrum:

    • Eliminative Physicalism: There are physical objects, but no minds or experiences.
    • Reductive physicalism: There are both, but only physical objects are fundamental: Minds and experiences are constituted by physical objects.
    • Dualism: There are both fundamental physical objects and fundamental minds and experiences.
    • Reductive Phenomenal Idealism: There are both, but only minds/experiences are fundamental: Physical objects are constituted by minds and experiences.
    • Eliminative Phenomenal Idealism: There are minds and experiences, but no physical objects.

Based on Hofweber’s definition and on that given by Foster (a contemporary defender of phenomenal idealism) it is fair to characterize Phenomenal Idealism for our purposes as just Reductive Phenomenal Idealism above. I located it on this spectrum to highlight the parallel between reductive phenomenal idealism and the reductive physicalism that we all know and love: At least at first glance, they both posit the same entities, and the same number of fundamental ontological kinds; the only difference is the direction of fundamentality.

Lewisian Phenomenal Idealism (LPI) has four components: An ontological claim, a philosophy of science claim, an empirical claim, and a revisionary reductive analysis of our talk about physical objects. It will take a while to explain each of them in sufficient detail:

LPI Ontology: Fundamentally, there are only minds and experiences.

I intend to avoid the issue of how minds and experiences are related—are they two separate kinds? Is one a property or relation of the other? It should be irrelevant for the purpose of this paper.

LPI Philosophy of Science: Laws of Nature are as David Lewis’ Best Systems Theory says: They are the theorems of the deductive system that describes the world with the best balance of simplicity, strength, and fit. (Lewis 1980)

This requires a bit of explanation. The idea is that which minds exist, and which sequences of experiences they have, can be described by a variety of deductive systems. Some are very detailed, accurately describing everything there is to describe, but are very complex; others are rather simple, but miss out on a few details. The best system is the hypothetical system that achieves the best balance of simplicity, strength, and fit. The Laws of Nature, then, are defined in terms of the best system. Lewis says they are the theorems of the best system; I’ll eventually go for something a little more nuanced, but that will do for now.

The Best Systems account of lawhood is controversial, but I’m happy to accept the consequences for now. If the worst that can be said of LPI is that it relies on Lewis’ theory of lawhood, this paper is a success.

LPI Empirical Claim: The Best System is of the following form: For each structure of type M in the theorems of [insert physicalist Best System here] passing through a sequence of states S1,…Sn, there exists a mind which has the sequence of experiences given by f(S1),…f(Sn).

This requires quite a lot of explanation. I’ll start by explaining what a physicalist Best System would look like, given what we know of physics. It would have some description of which objects in which relations exist in the initial condition, and then some description of how the system evolves over time. Both descriptions would be heavily constituted by the deliverances of ideal physics. It is this description, this Best System, that LPI places in the brackets above. Importantly, I described things this way for convenience only. LPI does not parasitically depend on physicalism; I put it that way for ease of exposition and to highlight the similarities between the two theories. Technically LPI would have [insert description of initial conditions and rules for how they evolve over time]; the idea is that whatever evidence physics gives us, according to physicalism, for the rules and conditions of the Best System is, according to LPI, evidence instead for the rules and conditions of the bracketed section of the Best System.

Now, reductive physicalism follows up its account of the Best System with a revisionary reductive analysis of mind/experiences, so that the theory as a whole can posit minds and experiences as well as fundamental physical objects. This reductive analysis is supposed to be partially the result of philosophy (armchair theorizing about what sorts of functional, behavioral, or structural properties a system must have in order to count as a mind) and partially the result of psychology (empirical data on which neurochemical structures lead to which experiences under which conditions). Never mind the details, the point is that according to reductive physicalism there is some type M such that a mind exists whenever there is a physical structure of type M. According to LPI, then, there is some type M such that a mind exists whenever in the bracketed section of the Best System there is a structure of type M. Something similar can be said about the function f. Whereas according to reductive physicalism, philosophy and psychology tell us how minds and experiences supervene on physical structures, according to LPI, philosophy and psychology tell us about sections of the Best System which describes which minds/experiences there are.

One important difference between LPI and reductive physicalism is that for reductive physicalism, all this stuff about minds and experiences is part of a revisionary reductive analysis, separate from the Best System for describing the world; by contrast, for LPI, this is all part of the Best System. More on this point in Section II.

LPI Reductive Analysis: Our talk about physical objects is analyzed as talk about the bracketed section of the Best System. Our talk about the Laws of Nature that the sciences discover is analyzed as talk about the bracketed section as well. Indeed for convenience we can think of the bracketed section as being a “simulation” of the world that the physicalist thinks is actual. Most ordinary conversation is analyzed as talk about the simulation.

I’ll supplement this with two examples: When a physicist says “We just discovered that there are exactly 13 kinds of fundamental particle,” their claim can be analyzed as “We just discovered that the part of the Best System which simulates a bunch of fundamental particles, simulates exactly 13 kinds.” When an atheist geologist says “There were rocks and planets for billions of years before there were minds,” their claim can be analyzed as “The part of the Best System which simulates a bunch of objects evolving over time, simulates rocks and planets billions of simulation-years before it simulates objects of type M.”

II: Discussion

Before moving on to Hofweber’s criticism of phenomenal idealism, it is worth taking the time to discuss a few issues that arise independently.

First, LPI may strike some readers as extremely suspicious. Isn’t it ad hoc? Doesn’t it basically steal all the achievements of reductive physicalism and claim them as its own? It seems like an accommodation, rather than a prediction, of the data.

I have three responses to this worry. The first is that there is nothing wrong with stealing the achievements of a successful theory, if all that means is making a new theory that combines the same predictive accuracy with additional desirable traits. Indeed in most cases when this happens, all the reasons we had for believing the old theory transfer over into reasons to believe the new theory. This is, in general, how human knowledge progresses.

My second point is a warning not to overstate the successes of reductive physicalism. Physics has been successful; geology has been successful; psychology and neuroscience have been successful, etc. But they would have been equally successful if LPI and not reductive physicalism had been the dominant ideology for the past two centuries.

My final point is that the empirical claim LPI makes actually flows fairly naturally from the Ontological and Philosophy of Science claims it makes. Our minds are extremely complex. If the Best System describing minds/experiences did so directly, by specifying which experiences were had, in which order, it would have to be similarly complex. Arguably this Best System would be more complex than the one posited by LPI’s Empirical Claim! By describing minds/experiences indirectly, by simulating a large physical universe that evolves according to simple laws, and then extracting descriptions of minds and experiences from that simulation, the Best System can compress the data significantly. Hence, arguably, LPI Empirical Claim is actually predicted by the conjunction of Occam’s Razor, the datum that complex minds exist, and the ontological and philosophy of science claims made by LPI. This puts LPI on the same footing as physicalism with respect to predicting the world as we find it.

All that being said, LPI is admittedly more complex than reductive physicalism. However complicated the Best System is according to reductive physicalism, the Best System according to LPI will be that and then some. (The additional complexity will be approximately the sum of the complexity of Type M and the complexity of the function f.) So, other things equal, by Occam’s Razor we ought to prefer reductive physicalism to LPI.

Other things are not equal. There are arguments against reductive physicalism, which, if successful, rule it out before Occam’s Razor can be brought to bear. These are the arguments that have occupied the literature on consciousness for decades and possibly centuries: the “explanatory gap,” the zombie thought experiment, the Mary’s Room case, etc.  I am not trying to say that these arguments are correct; my point is just that LPI can’t be ruled out straightaway by Occam’s Razor. The arguments must be considered first.

Finally, I suspect that LPI may solve many of the problems in philosophy of mind. It seems to provide a non-arbitrary connection between the mental and the physical, and explains how that connection holds. It accounts for our intuitions about qualia, the unity of consciousness, etc. It resolves worries about vagueness and borderline cases of consciousness/personhood. It even suggests a principled answer to questions about the persistence of personal identity over time! It does this in the same way that dualism does it—by positing fundamental minds/experiences subject to contingent laws. Questions about minds and experiences become normal empirical questions, no more mysterious than questions about the gravitational constant or the composition of water. Unlike dualism, though, LPI doesn’t run into problems trying to connect mind and matter—there is no overdetermination, no causal inaccessibility, etc.

I should qualify the above by saying that I haven’t worked out the implications of LPI in any detail yet; I may be wrong about its advantages. My point in mentioning these speculative ideas is to garner support for the thesis of this paper, which is that LPI—and by extension, phenomenal idealism more generally—shouldn’t be ruled out straightaway as conflicting with what we know to be true. On the contrary, it’s an interesting philosophical theory that deserves proper philosophical consideration. In the next three sections, I’ll lay out and discuss Hofweber’s arguments against this thesis.

III: The Coherence Constraint

In this section I will lay out Hofweber’s argument against phenomenal idealism in detail. According to my categorization, he has four arguments for why phenomenal idealism conflicts with what we know to be true. They aren’t meant to all apply at the same time; different arguments apply to different versions of phenomenal idealism. In this section I will show that LPI avoids the first three arguments. The remaining argument, which I take to be the most important, will occupy the rest of the paper.

Hofweber’s arguments revolve around the Coherence Constraint:

Coherence Constraint: An acceptable form of idealism must be coherent with what we generally take ourselves to know to be true. (p12)

Presumably the Coherence Constraint is to be generalized to all metaphysical theories, not just idealism. A helpful gloss on it follows:

To accept the coherence constraint is closely tied to thinking of metaphysics as being modest with respect to other authoritative domains of inquiry. Although any scientific result might be mistaken after all, metaphysics should not require for its work that this turns out to be so. It should do something over and above what other authoritative parts of inquiry do. It asks questions whose answer is not given by and not immediately implied by the results of the mature sciences. Applying this to idealism it means that idealism must be compatible with us living on a small planet in a galaxy among many. It must be compatible with our having a material body in a material world, and so on. It can make proposal about what matter is, metaphysically, or other things, but whatever it is supposed to be, it must be compatible with and coherent with the rest. This is the coherence constraint, and any form of idealism worth taking seriously must meet it. (p12)

I’ll have a lot to say about this in the next two sections. For now, I’ll briefly discuss how LPI avoids the first three arguments:

Argument One: Problem of Intersubjectivity

The idea behind this problem is that one of the things we know to be true is that there are other minds like us, and that their experiences are coordinated with ours—when I type the word “Porcupine” on my screen, I know that eventually someone (In this case Hofweber himself) will see the word “Porcupine” on his. On theistic versions of phenomenal idealism, this can easily be explained: God coordinates everyone’s experiences so that they match up in this way. But LPI is not theistic; can it explain how all our experiences are coordinated?

Yes it can. The Laws which govern minds and experiences involve a “simulation” of physical objects, and which minds there are and which experiences they have are determined by this simulation. If minds had uncoordinated experiences, the laws governing them would not be of the form specified by LPI Empirical Claim, and indeed would probably be much more complicated.

There is a legitimate worry about how LPI can explain the coordination of our experiences by reference to these Laws, since the Best System in which they are based is merely a description of which minds there are and which experiences they have. However this worry is a general problem with Lewisian accounts of lawhood, and not a problem with LPI in particular.

Argument Two: Problem of Unobserved Objects

The idea behind this problem is that one of the things we know to be true is that there were rocks and planets before there were minds—geology and astronomy have discovered this, at least if we assume atheism is true and there are no parallel universes with aliens in them etc. More generally, it seems like one of the things we know to be true is that there are objects which have not, are not, and never will be observed by any mind. How does LPI account for this fact?

It does so with its revisionary reductive analysis of objects. Talk about physical objects is analyzed as talk about structures in the simulation. When an atheist geologist says “There were rocks and planets for billions of years before there were minds,” their claim is analyzed as “The part of the Best System which simulates a bunch of objects evolving over time, simulates rocks and planets billions of simulation-years before it simulates objects of type M.” When an atheist physicist says “There are objects which have not, are not, and never will be observed by any mind” their claim is analyzed as “In the simulation, there are objects which never get observed by any mind.”

In general, LPI marks as true all the same sentences that ordinary physicalism marks as true—sentences like “There were rocks before there were minds,” “Some objects never get observed,” “There is a computer in front of me right now,” etc. Of course, LPI achieves this only by changing the referents of the relevant sentences: instead of referring to external, mind-independent objects, they refer to structures in a simulation, which is itself a component of the Best System for describing which minds/experiences there are. This is exactly what one should expect; of course a reductive phenomenal idealist theory is going to say that our talk about physical objects does not refer to external, mind-independent things, since reductive phenomenal idealism by definition says that physical objects are reduced to mind-dependent things. Other reductive phenomenal idealist theories reduced physical objects to counterfactual experiences, or possible experiences; LPI does something different, but similar in kind.

Nevertheless you might think that this changing-of-the-referents, this reductive analysis, is illegitimate. This is precisely the move made by Argument Four, and responding to it will occupy Sections IV and V of this paper.

Argument Three: Not Really Phenomenal Idealism

At various points in his book, Hofweber considers versions of phenomenal idealism that perhaps escape his criticisms—except that they do so at the cost of their phenomenal idealist credentials. One example is a version of pantheism, which says that physical objects are aspects of god—in particular, ideas in God’s mind. (p24) In this case, Hofweber says, physical objects inhabit a world that is independent of our minds at least, even though it might not be independent of all minds. It isn’t physicalism, but it isn’t phenomenal idealism either. Perhaps LPI has done the same; perhaps it avoids Hofweber’s arguments against phenomenal idealism because it is not really phenomenal idealism.

This argument is one of the reasons that I decided to build LPI around a Lewisian conception of lawhood. I can see how, arguably, if laws were self-sufficient entities in some sense, one might say that the “simulation” is really a mind-independent external reality. But since according to LPI physical objects are just structures in the simulation, and the simulation is just a part of the Best System, and the Best System is just a description of which minds there are and which experiences they have, physical objects really are mind-dependent according to LPI. And they are dependent on minds like ours, since minds like ours are the only minds there are.

IV: The No Reinterpretations Objection

The No Reinterpretations Objection revives the Problem of Unobserved Objects by rejecting LPI’s revisionary reductive analysis of our talk about physical objects:

The idealist must meet the coherence constraint by making clear that what their view is is coherent with what we know to be true. It is not enough to do this to assign some meaning to the words ‘there were dinosaurs’ and show that with this meaning it is compatible with idealism that ‘there were dinosaurs’ is true. Instead, the idealist must show that the statement is compatible with idealism with the meaning it in fact has, i.e. they must show that there were dinosaurs (before there were humans) is compatible with idealism. As such the idealists proposal about the meaning of `there were dinosaurs’ must be taken as an empirical proposal about the actual meaning of this sentence in English. It is one thing to show that idealism is compatible with the truth of this sentence given some assignment of meaning to this sentence, and quite another to show it is compatible with the truth of the sentence with the actual meaning of it. (p26)

As I understand it, Hofweber is objecting to the “Revisionary” part of LPI’s revisionary reductive analysis. He thinks that statements about physical objects only cohere with LPI if they do so given the meanings that they in fact have, in English. It doesn’t count if they cohere with LPI given different meanings supplied by LPI. On the face of it, this objection is very plausible. Any set of sentences can be made coherent with any other set of sentences, if you are allowed to change the meanings of the sentences! It seems that LPI is helping itself to a technique—revisionary reductive analysis—that will allow it to get away with anything.

In this section, I will respond to this basic version of the No Reinterpretations objection. I’ll argue that, in fact, the burden of proof is on Hofweber to go beyond what has been said so far and present a positive argument for the illegitimacy of LPI’s revisionary analysis. In the next section, I’ll examine a later argument Hofweber makes that I construe as an attempt to meet that burden.

I’ll begin by pointing out a tension between endorsing the Coherence Constraint and critiquing LPI in this way. In his elaboration quoted above, Hofweber explicitly permits idealism to “say what matter is, metaphysically.” (p12) Thus Hofweber agrees there is some legitimate middle ground between the bad kinds of revisionary analysis and mere proposals about the English meanings of words. My point so far is that it is unclear where the boundary is; in the remainder of this section, I’ll argue that LPI occupies that acceptable middle ground.

Consideration One:

My first point is that revisionary reductive analysis is a legitimate philosophical and scientific move that has been made successfully many times in the past, and is often attempted today. I’ll give three examples, but many more could be generated if necessary:

    • Laws Example: Laws of Nature were once taken more literally, as decrees of God, or at least descriptions of His governance of creation. At some point the scientific community switched to a new concept that didn’t necessitate a Lawgiver.
    • Relativity Example: Space and time are intuitively thought of as distinct; even among most normal English speakers today, it is fair to say that space means Euclidian space and time means something else. Yet according to relativity, there is no such space and no such time in the actual world.
    • Physicalism Example: Minds were for the longest time thought to be essentially immaterial and unified. This was no “folk” belief either—the best scientists and philosophers of the modern era thought this. Reductive physicalism eventually came along and overturned this view, saying that minds are neither immaterial nor unified.

I think that the Physicalism example is particularly close to what LPI is trying to do: Recalling the physicalism-dualism-idealism spectrum, LPI is advocating that we revise our understanding of the world in the same way that Physicalism did, only in the opposite direction.

I hold these up as examples of revisionary analysis, but this can be disputed. For example, one might say instead that the meaning of “minds” stayed the same, because it was never committed to immateriality and unity in the first place. Rather, “minds” meant something like “That in virtue of which things can think, feel, and choose” or perhaps “The relevant cause of my ‘mind’ mental representation tokens.”

I’m happy to accept this possibility, because if we can convince ourselves that the meaning of our words was preserved in the examples given, i.e. if we can convince ourselves that they aren’t really examples of revisionary analyses, then we should also be able to convince ourselves that LPI isn’t really performing a revisionary analysis either. Whatever it is that Physicalism did when it convinced people that minds weren’t really immaterial unities—I claim that LPI is trying to do the same thing, but in the opposite direction.

It’s worth mentioning a point made by Foster and Smithson: Often when people adopt a radical change in their ontological beliefs, they initially express the change by saying “I now believe that there is no X!” yet then soon after start saying “I still believe there is X, but it isn’t what I used to think it was.” (Foster 1994, Smithson 2015) I remember some of my teachers saying things like this in middle school, when we learned about how material objects are “mostly empty space,” and about how they “never actually touch each other,” etc. Presumably the same thing happened with the examples given above, and presumably the same thing would happen again if we started believing in LPI. I take this as evidence that the sort of thing LPI is doing is closer to “say[ing] what matter is, metaphysically” than to “conflicting with what we know to be true.” (Hofweber p12) It is evidence that our cherished knowledge of physical objects is not knowledge that there are physical-objects-as-physicalists-define-them, but rather knowledge that there are physical objects-according-to-some-broader-definition.

Consideration Two: The rough idea here is that we want our epistemology to be reasonably open to change, so that we can converge on the truth from a wide variety of starting points. We don’t want which beliefs are permissible to depend on non-truth-tracking historical accidents. To illustrate:

LPI Takeover Case: Suppose that I convert some wealthy donors to belief in LPI, and we engage in a decades-long covert proselytization process in which we change the way young scientists are taught to think about physical objects. We get the textbooks changed to use definitions of “object” that are neutral between the LPI definition and the physicalist definition, etc. In other words, we change the meaning of the term “object” in English, so that it is more permissive. Once we have accomplished this, we introduce LPI into the philosophical discourse, confident that it now passes the Coherence Constraint.

Clearly something has gone wrong here. Yet this is, I claim, relevantly similar to what happened with reductive physicalism. Not that there was a conspiracy or anything; just that the meanings of our terms shifted over time, in a way that allowed for the possibility of reductive physicalism, but not in a way that particularly tracked the truth—it’s not like the meanings of our terms shifted because we found empirical evidence that reductive physicalism (as opposed to dualism) was true. Perhaps the shift was motivated by philosophical considerations; people became dissatisfied with dualism as an explanation for mental causation, etc. But in that case, the conclusion should be that the shift in meaning in the LPI Takeover Case is legitimate if there are good philosophical reasons motivating it. And if that’s the case, then we can’t yet rule out LPI as conflicting with what we know to be true, because we have to consider the philosophical arguments for and against it first. And that’s precisely the thesis of this paper.

Consideration Three: Contemporary analytic philosophy is full of attempted revisionary analyses. Philosophers debate definitions of objective chance, universals, mathematical objects, composite objects, vague objects, etc. and they debate whether or not these things exist, and if so, under what definition. Sometimes attention is paid to the actual English meanings of our terms, but often it is not. One hears things like “There are no chairs, only particles arranged chair-wise,” and “There aren’t really any numbers; they are just convenient fictions.” These are examples of the english meanings being kept but the associated knowledge claims rejected. The examples I gave earlier are examples of the opposite strategy. Thus, LPI is very much situated within a broader tradition; what it is doing is normal and accepted.

Maintaining his consistency, Hofweber goes on to explicitly reject some of the attempts I mention above—specifically, nominalism in mathematics and eliminativism about ordinary objects. (Chapter 7) His rejection of LPI’s revisionary analysis is part of a broader rejection of a whole class of similar analyses in contemporary analytic philosophy.

In the next section I will evaluate his argument for this; for now I stick to the more moderate conclusion—which I take to have established by now—that the burden of proof is on Hofweber: In the absence of a good positive argument against the legitimacy of LPI’s revisionary analysis, we ought to accord it the same legitimacy as all the other historically successful and currently accepted attempts at revisionary analysis. This doesn’t mean we should accept LPI, of course—it just means that LPI should be accepted or rejected on the basis of all the arguments for and against it, and not simply rejected as conflicting with what we know to be true.

V: The Argument Against Reinterpretation

Elsewhere in his book (Chapter 7), Hofweber presents an argument for the existence of ordinary composite objects and against certain views in mereology, which probably count as “Eliminative Physicalism” under my classification.  I believe this argument can be generalized to apply to the case of LPI as well. In this section I’ll first lay out the argument as I construe it, and then attempt to poke three holes in it.

Hofweber’s Argument:

    1. (P1) Skepticism is false. (p251)
    2. (P2) If skepticism is false, then we are defeasibly entitled to the beliefs we form on the basis of perception. (p251)
    3. (L3) (From P1 & P2) We are defeasibly entitled to the beliefs we form on the basis of perception. (p251)
    4. (P4) Some of the beliefs we form on the basis of perception are incompatible with LPI. (p252)
    5. (L5) (From L3 and P4) So there are some beliefs which we are defeasibly entitled to, which are incompatible with LPI. (p252)
    6. (P6) If our entitlement to beliefs is defeated, it is either rebutted or undercut, and if it is undercut, either our entitlement to all perceptual beliefs is undercut or not. (p252)
    7. (P7) If our entitlement to all perceptual beliefs is undercut, then skepticism is true. (p253)
    8. (L8) (From P1 & P7) So it’s not the case that our entitlement to all perceptual beliefs is undercut. (p253)
    9. (P9) (With support from L8) Our entitlement to the beliefs mentioned in L5 (the ones that are incompatible with LPI) is not undercut. (p254)
    10. (P10) If our entitlement to those beliefs is not undercut, we have overwhelming empirical evidence for them. (p258)
    11. (L11) (From L5, P9, & P10) So there are beliefs formed on the basis of perception which are incompatible with LPI, which are not undercut, and which we have overwhelming empirical evidence for. (p257-261)
    12. (P12) Beliefs for which we have overwhelming empirical evidence cannot be rebutted. (p261-263)
    13. (C13) (From L3, L11 & P12) So there are beliefs incompatible with LPI, for which we have overwhelming empirical evidence, and we are entitled to those beliefs. (p263)

Since the original argument is about composite objects rather than mind-independent objects, I’ve had to modify a few of the premises. In particular, the relevant beliefs posited by P4 as incompatible with LPI are things like “There were rocks before there were humans,” and “I see a rock over there right now.” Crucially, terms like “rock” in these beliefs are supposed to be definitively incompatible with LPI. It’s not merely that terms like “rock” don’t mean “rock-structure in the LPI simulation;” they have to also not mean “whatever usually causes rock-appearances” or “the most natural existing referent for my use of the term ‘rock,’” or “something, I know not what, which I am justified in believing exists when I am appeared to rockly.” None of these meanings, even though they are neutral between physicalism and idealism, will do. P4 is saying that we form beliefs on the basis of perception which are not neutral between LPI and physicalism; rather, they are definitively incompatible with LPI.

I will accept P4, with qualifications: Given the above, these beliefs are probably not universally shared; many people probably form beliefs on the basis of perception that are weak enough to be compatible with LPI. Moreover, as Hofweber admits, (p254) these beliefs are contingent: If our culture, education system, and/or brain chemistry were different, we would instead form other, very similar beliefs that would allow us to function in all the same ways without being incompatible with LPI. Finally, LPI is far from alone as a theory which many people form perceptual beliefs against: Reductive physicalism and atheism, for example, are both theories which are incompatible with the perceptual beliefs of many (and perhaps even most) people.

The two premises I find problematic are the ones that Hofweber spends the most time justifying: P9 and P10. In the remainder of this section, I’ll lay out each of his arguments and explain why I don’t think they work.

Premise 9: (With support from L8) Our entitlement to the beliefs mentioned in L5 (the ones that are incompatible with LPI) is not undercut.

Hofweber says (and I agree) that the issue of undercutting in this case comes down to whether or not we have reason to think that the aforementioned beliefs track the truth. Hofweber considers an argument that (in the case of object composition, which his argument is geared towards) they don’t: In the possible world in which there are only simples arranged cup-wise, but no cup, we would still form the same perceptual beliefs in cups. So our belief in cups doesn’t track the truth of whether or not there are cups. (p254) Hofweber responds thus:

The counterfactual situation relevant to evaluate the counterfactual conditional about my cup not being there isn’t one where the simples are still there, but they somehow don’t compose a cup, but rather one where both the simples and the cup are gone. But then nothing would cause me to believe in a cup in front of me, and so my cup beliefs track the cup facts: all things being equal, I wouldn’t have that belief if there wasn’t a cup. (p255)

Converting this argument to work against LPI, Hofweber would presumably say that the beliefs we form on the basis of perception (and which are incompatible with LPI) track the truth, and that they do so because the relevant counterfactual in which they are false is the one in which e.g. we aren’t looking at anything that looks like a rock, geology finds human bones older than any rock, etc. In that counterfactual, we would not form those beliefs.

I am troubled by this reasoning. It seems to me that both counterfactuals are relevant; a belief can track the truth of some propositions but not others. It seems natural to say that our belief that there is a rock (in the LPI-incompatible sense) directly ahead is counterfactually able to distinguish between worlds in which there is a rock and worlds in which there is open space, but not between worlds in which there is a rock (in the LPI-incompatible sense) and worlds in which there is a rock (in the LPI-analysis sense). So, the thought goes, our belief is partially undercut—the part that has to do with the metaphysical nature of the rock is undercut, but the part that has to do with pretty much everything else, like what will happen if we kick the rock, how we can best avoid the rock, etc. is not. This would rescue LPI.

I expect Hofweber’s response to be that that way lies skepticism; we can extend that line of reasoning to the conclusion that all of our perceptual beliefs are systematically undercut, since there are worlds in which e.g. an evil demon is making us hallucinate everything. (p255)

I have two responses to this. The first is that we might be able to use “prior plausibility” to distinguish between the radical skeptical hypotheses and the non-radical, benignly skeptical hypotheses like LPI, mereological nihilism, and reductive physicalism. Presumably these latter hypotheses have more prior plausibility than the evil demon hypothesis. At any rate, in order to settle just how plausible these hypotheses are exactly, we have to consider the arguments for and against them. So if Hofweber’s argument that LPI shouldn’t be taken seriously depends on an assignment of low prior probability to LPI, then he is begging the question; he must instead first address the philosophical arguments for and against LPI, which is exactly the thesis of this paper.

My second response is that some of the examples I gave in Section IV seem to be similar enough to LPI that Hofweber’s argument here would work against them as well! I think any of them would do, but I’ll go with my favorite: Reductive physicalism. Most of the smartest philosophers and scientists in the 17th century formed beliefs on the basis of perception that were incompatible with reductive physicalism; for example, they thought that minds were immaterial and unified. They could use Hofweber’s argument to say that their beliefs track the truth: The relevant counterfactual in which their beliefs are false is one in which minds don’t even exist, not one in which minds exist but are somehow composed of material objects.

Similarly, one can imagine early scientists who thought of Laws of Nature as being given by a divine Lawgiver saying: “The relevant counterfactual in which there is no Lawgiver is a world in which everything moves about higgledy-piggledy; hence my belief in a Lawgiver on the basis of the regularity in the universe tracks the truth.” Indeed I have actually met people who argued this.

Given what I said in Section IV about how our beliefs oughtn’t depend on historical accidents (Consideration Two) I take these examples to be reductios of Hofweber’s argument.

Premise 10: If our entitlement to those beliefs is not undercut, we have overwhelming empirical evidence for them.

Hofweber takes himself to have established by this point that we have non-undercut entitlement to our beliefs about ordinary objects, but not to beliefs about mere simples arranged object-wise (since we don’t in fact have perceptual beliefs in such things). (p256) Converting this into talk about LPI, we may suppose that Hofweber has established by this point that we have non-undercut entitlement to our beliefs about mind-independent rocks, but not to our beliefs about more-broadly-defined-rocks, because we don’t in fact form the latter beliefs on the basis of perception.

Continuing to describe the converted argument rather than the actual one: Hofweber goes on to argue that this disanalogy compounds constantly into a tremendous empirical success for physicalism and predictive failure for LPI. LPI predicts that there are no mind-independent rocks in front of us; yet perception gives us non-undercut entitlement to believe that there are, every day. Even though LPI might in some sense predict our phenomenal experience just as well as the alternatives, it doesn’t predict certain crucial facts like the above.

As a result, Hofweber claims, science heavily disconfirms LPI: Our current scientific theories are defined using terminology that is inconsistent with LPI, and they are far more predictively accurate than their counterparts that would be defined using LPI’s revisionary analyses. (p261)

Hofweber goes on to say that, given the immense body of scientific and perceptual evidence against LPI, no amount of metaphysical argumentation should convince us that LPI is true. (p263) We can rule it out as conflicting with what we know to be true.

I find this argument problematic as well. It starts with a fairly innocuous premise about an asymmetry in our beliefs about two opposing theories, and ends up with a very strong and surprising conclusion. It smells suspiciously like the “Easy Knowledge/Bootstrapping” case described by Vogel and Cohen. Of course that isn’t a legitimate objection, so here are two:

My first objection is that Hofweber seems to be assuming a very strong form of scientific realism here.  Even if we set aside the anti-realists (though they constitute 11.6% of the professional population) the positions that remain, realism and structural realism, generally don’t say that we have overwhelming evidence for the literal truth of our current best theories. Rather, they say that we should accept them for now, while assigning high credence to the possibility that the truth is more nuanced and even that the central terms in our current best theories fail to refer. This caution is motivated by the history of science, which is a history of theory after well-confirmed theory turning out to be false when strictly and literally interpreted, even to the point where their central terms fail to refer. Granted, my worry here only applies to the scientific component of Hofweber’s argument, but I think it generalizes to the basic perceptual component as well. The history of strictly-literally-interpreted theories about ordinary observable objects mirrors the history of strictly-literally-interpreted scientific theories about unobservables.

My second objection is that the mere accumulation of truth-tracking, non-undercut perceptual beliefs does not by itself confer overwhelming evidence. Consider a doctor who examines a patient and comes to the perceptual belief, based on her years of experience, that the patient has a certain rare cancer. Suppose that the doctor knows her perceptions track the truth in cases like these: She gets the right answer 99% of the time. Does she have overwhelming evidence that the patient has the rare cancer? Does continuing to examine the patient again and again give her any more evidence?

The answer to both questions is no, if the cancer is sufficiently rare and if her repeated examinations are not independent. Suppose, as often happens, that the cancer strikes only one in every ten thousand patients, and that her repeated examinations are going to yield the same result, since they are based on the same perceived symptoms and intuitive judgments. Then no matter how many times the doctor examines the patient, her credence that the patient has the rare cancer should not rise above 0.01.

The moral of this story is that prior probabilities/base rates are crucial. Depending on how we like to put it, we can say either that priors are crucial for determining whether or not a reliable belief is undercut, or we can say that they are crucial for determining whether a non-undercut reliable belief constitutes overwhelming evidence. Either way, the lesson is that Hofweber needs to say something about LPI’s prior probability in order for his argument to work. But this will involve examining and engaging with the philosophical arguments for and against LPI, exactly as this paper urges.

VI: Conclusion

I admit that the arguments I have given do not conclusively defend LPI; phenomenal idealism may in fact turn out to violate the Coherence Constraint. This admission comes not from any particular weakness I have identified in my reasoning, but rather from a general worry that I don’t understand Hofweber’s: I’m not used to thinking in terms of entitlement, defeaters, and undercutting; my experience has more to do with credences, priors, and Bayesian updating.

Nevertheless, I think that the arguments presented here are more than strong enough to establish my modest conclusion: that LPI ought not be ruled out as conflicting with what we known to be true. On the contrary, it should be taken as seriously as any other weird philosophical theory, and the arguments for and against it should be explored. We should believe this until proven otherwise; the burden of proof is on Hofweber to do so, and he has not yet met it.


Foster, J. (1994) ‘In Defense of Phenomenalistic Idealism.’ Philosophy and Phenomenological Research, Vol. 54, No. 3. September. pp. 509-529

Goldman, A and Beddor, B. (2015) “Reliabilist Epistemology”, The Stanford Encyclopedia of Philosophy (Winter 2015 Edition), Edward N. Zalta (ed.), forthcoming URL = <;.

Hofweber, T. (2015) Idealism and the Limits of Conceptual Representation. Unpublished draft. The relevant chapters are 1, 2, and 7.

Hofweber, T. (2009) ‘Ambitious, Yet Modest, Metaphysics.’ In Chalmers, Manley & Wasserman (eds.), Metametaphysics: New Essays on the Foundations of Ontology. Oxford University Press 260—289

Ladyman, J. (1998) What is Structural Realism? (Studies in the History and Philosophy of Science vol 29 No 3, pp 409-424.

Lewis, D. (1994) Humean Supervenience Debugged. Mind, Vol. 103.412.

Lewis, D. (1980) A Subjectivist’s Guide to Objective Chance, in R. Jeffrey, ed., Studies in Inductive Logic and Probability, vol II. Berkeley: University of California.

Planck, M. (1996) ‘The Universe in the Light of Modern Physics’, in W. Schirmacher (ed.), German Essays on Science in the 20th Century (New York: Continuum), pp. 38–57.

Roberts, J. (2014) ‘Humean Laws and the Power to Explain.’ For UNC-Hebrew University workshop. Available online at

Stanford, K. (2001) ‘Refusing the Devil’s Bargain: What Kind of Underdetermination Should We Take Seriously?’ Philosophy of Science, Vol. 68, No. 3, Supplement: Proceedings of the 2000 Biennial Meeting of the Philosophy of Science Association. Part I: Contributed Papers (Sep., 2001), pp. S1-S12

Smithson, R. (2015) ‘Edenic Idealism.’ Unpublished draft.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s