David Chalmers’ Fixation With Logical Possibility

What’s the point of thinking up six logical possibilities before each breakfast? (Thanks to Lewis Carroll for this question.)

In David Chalmers’ book The Conscious Mind, he introduces a logical possibility on almost every page. In it, he talks about “an angel world”, “flying telephones”, “ectoplasm” and a monkey who writes Hamlet.

Admittedly, most of these logical possibilities are of little interest to Chalmers. That’s either because they’re hugely improbable or because they don’t help him philosophically. It’s primarily (philosophical) zombies which he specializes in — and it is these logically-possible creatures which help.

Not only is Chalmers’ keenness on logical possibilities a philosophical position, but logical possibilities also help him advance one of his positions within philosophy. To quote Chalmers himself:

“I am not the first to use the argument from logical possibility against materialism. Indeed, I think that in one form or another it is the fundamental anti-materialist argument in the philosophy of mind.”

Chalmers and other “anti-materialists” require logical possibilities in order to advance anti-materialism. In other words, their arguments only work if one countenances all sorts of strange logical possibilities. This isn’t to say that materialists (as well as other kinds of philosophers) don’t themselves rely on logical possibilities in various ways — though they don’t do so nearly so often as philosophers like Chalmers.

Chalmers is explicit about his philosophical use of logical possibilities when he says that

“the question is not whether it is plausible that zombies could exist in our world, or even whether the idea of a zombie is a natural one; the question is whether the notion of a zombie is conceptually coherent”.

Thus if the state of affairs A (or proposition P) is “conceptually coherent”, then A (or P) is also logically possible. All this is tied together by these three statements:

1) That which is conceivable is therefore also logically possible. 2) State of affairs A (or proposition P) is conceivable because conceptually coherent. 3) And state of affairs A (or proposition P) is conceptually coherent because it is conceivable.

So when a philosopher cites various logical possibilities, all sorts of philosophical positions become available… or possible. That’s primarily because once a given philosopher sets a particular logical possibility among the pigeons, then it’s surely the duty of other philosophers to tackle that logical possibility. And if they don’t, then surely they’re philosophical philistines… or “verificationists”. Indeed Chalmers lays his cards on the table when he states this:

“In general, a certain burden of proof lies on those who claim that a given description is logically impossible.”

This means that there’s a lot of weight attached to logical possibilities. The importance of logical possibilities is made clear when Chalmers cites one specific example:

“So even if a zombie world is conceivable only in the sense in which it conceivable that water is not H2O, that is enough to establish that consciousness cannot be reductively explained.”

In other words, logical possibilities (as well as — as it were — “conceivables”) serve a larger philosophical purpose.

My points above may seem too strong. Nonetheless, Chalmers did say that

“the argument from logical possibility [is] the fundamental anti-materialist argument in the philosophy of mind”.

In other words, citing logical possibilities isn’t just another tool — it is the tool to fight materialism (perhaps much more too). Yet Chalmers suggests that we shouldn’t get “too worried about odd things that happen in logically possible worlds”. However, he then immediately takes that back by saying that “there is room to be perturbed by what is going on”.

Six Logical Possibilities Before Each Breakfast

David Chalmers says that there are innumerable logical possibilities. And partly because of that, Bertrand Russell once wrote the following:

“No logical absurdity results from the hypothesis that the world consists of nothing but myself… and that everything else is mere fancy.”

Yet, despite writing the above, Russell went on to say that

“[b]ut although this is not logically impossible, there is no reason whatever to suppose that it is true; and it is, in fact, a less simple hypothesis, viewed as a means of accounting for the facts of our own life, than the commonsense hypothesis that there really are objects independent of us, whose action on us causes our sensations”.

So we need to be given philosophical reasons as to why we should consider these (or other) logical possibilities — otherwise, philosophers may end up spending their entire lives considering every logical possibility they can’t (as the philosopher David Lewis put it) “properly ignore”.

So if I carry on with Russell’s theme:

1) When I wake up tomorrow morning, it’s logically possible that I will still be asleep. 2) If I do genuinely wake up tomorrow morning (how could I know?), then it will be logically possible that the world’s entire population will be dead. 3) And when I then move over to the tap, it will be logically possible that poison — not water — could come out of it. 4) If it were to be genuine water, then it will also be logically possible that I could choke on that water. 5) If I then look out of the window, it will logically possible that the town I see is a simulation of what I saw the day before... And so on and so on and so on.

In other words, I could think of six logical possibilities before each breakfast.

As with Russell, should I say that “although [they are] not logically impossible, there is no reason whatever to suppose that [they are] true”?

Russell’s next point is that the logically-possible hypothesis is (often?) “less simple” than the every day (or common sense) one. He told us that

“It is, in fact, a less simple hypothesis, viewed as a means of accounting for the facts of our own life”.

In my examples, this means that:

1) It is a less simple hypothesis to believe that I’m currently dreaming. 2) It is a less simple hypothesis to believe that the entire world population is now dead. 3) It is a less simple hypothesis to believe that poison (rather than water) will come out of my tap. 4) It is a less simple hypothesis to believe that the water I drink will choke me. 5) It is a less simple hypothesis to believe that my window-view is a mere simulation of the facts.

Indeed isn’t it also less simple to believe in Chalmers’ zombies?

However, Chalmers doesn’t care about that because the logical possibility of zombies is a means to a philosophical end.

Despite that, Chalmers appears to question his own use of logical possibilities when he uses the word “plausible”. That is, an indefinite number of things is logically possible — but how plausible (as well as probable) are they?

Thus all Chalmers’ own logical possibilities are “logically compatible with the data”. However, “this is not enough to make them plausible”. To be more precise, this talk of plausibility is set within the well-known context that

“[f]or any scientific theory one can easily construct as ad hoc hypothesis that is empirically equivalent”.

In other words, as with scientific theories, many of Chalmers’ logical possibilities are of the kind that there can be no empirical way of establishing either their truth or falsity. They’re literally beyond the empirical or observational. That’s primarily because possibilities aren’t even meant to be either true or false. Possibilities are simply meant to be… possible. Then again, logical possibilities could become true or false — it’s just that they aren’t true or false.

René Descartes

Much of this talk about logical possibilities goes back to Descartes and his own arguments from logical possibility; which have inspired Chalmers (even if he updates them) and many other contemporary philosophers. Chalmers writes:

“[S]ome will find the argument for dualism that I have given reminiscent of the argument given by Descartes. Descartes argued that he could imagine his mind existing separately from his body, so his mind could not be identical to his body.”

As with my questions about Chalmers’ conceivings (or conceivables): what, exactly, did Descartes imagine?

Of course, the question of the status of imaginability in philosophical reasoning is often asked. What isn’t often asked is what, exactly, is being imagined in the first place.

This may remind readers of Bishop Berkeley’s argument that those who imagine an event that occurs without observers are actually imagining what it would be like from the observer’s perspective (e.g. when a tree is falling down).

That is, we imagine some kind of disembodied mind (if with sensory receptors) observing the event. (Of course, the idea of a disembodied mind observing a tree falling down is ironic when set within the context of this discussion about Descartes’ focus on imaginability and his belief in mind-body dualism.)

So did Descartes really imagine a disembodied mind? If he did, then what form did it take? Can a disembodied mind experience anything or does it simply indulge in pure thought? What kind of pure thought would that be? (Descartes did exclude sensations and the senses from the mind — see here.)

In any case, Chalmers then goes on to say that

“[t]his sort of argument [i.e., Descartes’] is generally regarded to be flawed: just because one can imagine that A and B are not identical, it does not follow that A and B are not identical (think of the Morning Star and the Evening Star, for example)”.

Again it can be asked whether or not Descartes did imagine a disembodied mind. If he didn’t, then this question of identity between A and B doesn’t even arise. Chalmers then makes the obvious point:

“Might not my argument make a similar mistake? The zombie world only shows that it is conceivable that one might have a physical state without consciousness; it does not show that a physical state and consciousness are not identical.”

So Chalmers offers his own Cartesian variant.

As it is, I simply can’t make sense of Chalmers’ riposte to (as it were) naïve Cartesianism. He writes:

“The form of the argument is not, ‘Once can imagine physical P.’ The form of the argument is rather, ‘One can imagine all the physical facts holding without the facts about consciousness holding, so the physical facts do not exhaust all the facts.’…”

To state the obvious, Chalmers is still relying on imagination here. In other words, is there really a big difference between:

“One can imagine a physical P”


“One can imagine all the physical facts holding without the facts about consciousness holding.”

It’s still imagination that holds the trump card in both cases. And it’s this reliance on imagination that’s being questioned in both cases. At first glance, the words “[o]ne can imagine all the physical facts holding without the facts about consciousness holding” hold no water. In fact, I have a problem knowing what it even means.

For example, if we invert the argument, what would we imagine if we imagined “all the physical facts holding” and consciousness holding too? How do you imagine the consciousness of another being or entity? Through its linguistic and physical behaviour? Well, Chalmers himself wouldn’t accept that because it would be what some people call “reductionist”. So, again, what would we be imagining?

This is also like the point made about monism and pluralism by A.J. Ayer. He said that if monism were true, then the world would be exactly the same as that experienced by a pluralist. Similarly, how would:

Imagining all the physical facts holding without consciousness

differ from

Imagining all the physical facts holding with the addition of consciousness?

Logical Possibilities as Tools

Despite all the above, it’s almost as if (at least at times) Chalmers wants to get logical possibility out of the way in order to deal with what he calls “empirical possibility”. Indeed he is explicit about this when he writes the following:

“It is useful to think of a logically possible world as a world that it would have been in God’s power (hypothetically!) to create, had he so chosen.”

In that sense, logical possibilities play the role that possible worlds do for those who don’t believe in their actual existence as “concrete” entities. (As David Lewis did when he used phrases like “worlds like our own”.)

The question is, then, what purpose do these logical possibilities and possible worlds play if they aren’t actually “actual”, “concrete” or instantiated? In the case of the possible world, much has been written about their efficacy for creating a semantics of modal terms, statements, and whatnot.

So one would assume that something similar follows for Chalmers’ own logical possibilities (such as zombies, “dancing qualia”, etc.). So it’s useful that Chalmers himself discussed his position on possible worlds. This means that what he says may provide us with a way in for discussing his position on logical possibility. Chalmers writes:

“I will not engage the vexed question of the ontological status of these worlds, but simply take them for granted as a tool…”

So I can rewrite that passage in the following manner:

I will not engage the vexed question of the ontological status of these logical possibilities, but simply take them for granted as a tool.

David Lewis (as hinted at a moment ago) asked cogent questions about all our talk of possibilities… and necessities. That is, rather than stress the fact that we can conceive of possibilities, Lewis was concerned with what logical and ontological status these possibilities have. Like “Plato’s Beard”, the very fact that we can conceive of them must surely mean that they have some kind of ontological status. According to Ted Sider’s David Lewis:

“We do have a reason to believe in his possible worlds: only by believing in them can we demystify necessity and possibility."

Sider also offers us a pragmatic take on possible worlds which can also be applied to logical possibilities. He tells us that “it is sometimes reasonable to postulate things for theoretical reasons”. For example, “no one has ever directly perceived an electron”; though physicists “postulate electrons to explain the results of the experiments they perform”.

Sider then goes on to say that certain philosophers believe that possibilities (such as unicorns or Tom Hanks being a serial killer) have some kind of being even though they aren’t actual. Are these possibilities what Sider calls “ghostly things”? Not really. That’s because ghosts are believed to exist by some people. They simply have the ghostly property of, say, intangibility. (In other words, ghosts aren’t meant to be abstract entities.) Or as Sider puts it:

“Rather than making it the case that unicorns are possible, the existence of a ghostly unicorn would just mean that ghostly things are actual.”

So how should we see Chalmers’ logical possibilities within the context just discussed? In Sider’s words, “if possibilities are not ghostly entities, then what are they?”. If possibilities, such as Tom Hanks being a serial killer, are not ghostlike or anything else, then what are we talking about when we talk about them? What is Tom Hanks the serial killer, Pegasus, or even the round square?

Indeed what philosophical and actual status do Chalmers’ zombies have?

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