The P-Zombie Argument

Thought Experiments [Part 2]

Imagine a zombie.

Not a flesh-eating monster that you might find in a Hollywood blockbuster (or the feature image of this article), but a philosophical zombie. A being just like you but with no conscious experience.

These hypothetical entities are the subject of one of philosophy's most talked about thought experiments.

The philosophical zombie argument is designed to challenge physicalism — the idea that consciousness can be fully explained by the physical.

Of all the thought experiments, zombies have perhaps sparked the most intense debates. So, I thought, why not make the first thought experiment in our series on thought experiments a big one!?

This week, let's explore the zombie thought experiment by asking three questions:

1. What exactly is a philosophical zombie?

2. Why might we find the zombie argument compelling? and,

3. What are the main objections to the zombie argument?

Q1: What Exactly is a Philosophical Zombie?

In philosopher-speak, a zombie is like a human doppelganger — identical down to the last atom and behaving just like you or me. The only thing that is different is that zombies lack conscious experiences. There is nothing that it is like to be a zombie.

If you stomp on a zombie’s toe, and they'll say, Ouch! — just like you would. They'll even insist they're hurting. They might even react as if they were annoyed that you stomped on their toe. But that’s all just automatic responses — there’s nothing actually going on inside — there’s no subjective ouchiness involved.

In Part 1 of this series, we talked about the conceivability implies possibility principle — also called the conceivability argument — if we can conceive of something, then it is possible, at least in some possible world.

Some philosophers, most notably David Chalmers, argue that we can conceive of zombies. According to the conceivability argument, this makes zombies possible. And, it is argued, if zombies are possible, physicalism must be false.

In Consciousness and its Place in Nature, Chalmers lays out the zombie argument as follows:

• Premise 1: It is conceivable that there be zombies

• Premise 2: If it is conceivable that there be zombies, it is metaphysically possible that there be zombies.

• Premise 3: If it is metaphysically possible that there be zombies, then consciousness is nonphysical.

  ---

  Conclusion: Consciousness is nonphysical.

It's important to note that the zombie argument doesn't claim or require that zombies exist in our world. The possibility of zombies, if granted, is enough to challenge physicalism.

As we discussed in Part 1, arguments based on the conceivability argument are controversial, and the zombie argument is no exception.

But before we get to the objections, let's consider what makes the zombie argument so influential.

Q2: Why Might We Find the Zombie Argument Compelling?

Imagine your philosophical zombie twin — physically identical to you and behaves exactly like you do, but without conscious experiences. In line with convention, we'll call this zombie twin a p-zombie.

Now, let's imagine a slightly different type of zombie twin. Like your p-zombie twin, this new type of zombie twin is physically identical to you and behaves exactly like you do. The only difference is that this zombie twin can run twice as fast as you can. Let's call this zombie a speed zombie (or s-zombie for short).

This s-zombie twin seems much harder to imagine than your p-zombie twin, doesn’t it!?

After all, if s-zombie is physically identical to you, how could it possibly run faster? The ability to run at a certain speed is directly tied to physical attributes like muscle strength, bone structure, and cardiovascular capacity. For s-zombie to run faster than you while remaining physically identical to you creates a logical contradiction that our minds struggle to reconcile.

Let's apply the conceivability argument here. We can't conceive of an s-zombie, so we conclude that s-zombies are not possible. But, the argument goes, we can conceive of a p-zombie. So, p-zombies must be possible. The reason s-zombies are not conceivable, while p-zombies are, is that physical properties like speed are inherent in physical structure, while consciousness, the argument goes, is not.

The zombie argument is an argument against physicalism — the view that everything in reality, including consciousness, can be fully explained by physical facts. For physicalism to be true, it's widely agreed that complete knowledge of the physical world should, in principle, give us knowledge of everything else — including consciousness. But if there are truths about consciousness that we can't conclude from physical truths (as the zombie argument suggests), then physicalism faces a significant challenge.

Q3: What Are the Main Objections to the Zombie Argument?

Like many philosophical arguments, objections to the zombie argument typically challenge one or more of its premises.

Let’s take each premise in turn.

Challenging Premise 1: Zombies Are Not Conceivable

At first glance, some ideas might seem perfectly conceivable. But when we dig deeper, we often realise we're not really conceiving what it is we are being asked to conceive.

When we first imagined our speed zombie twin, perhaps, at least for a few seconds, an s-zombie might have seemed conceivable. But with some more thought, we realise it doesn’t make much sense — our initial conception of an s-zombie was not adequately comprehensive.

Some philosophers claim the zombie argument falls into the same trap. They argue that if we think about it long enough, we'd see that the concept of p-zombies doesn't make much sense. They might feel conceivable, but that’s because we don’t know enough about what it would mean for a p-zombie to exist. There is still so much that we don’t know about the physical world.

As science learns more, we might discover that the idea of a consciousness-free copy of ourselves just doesn't add up. It might be that we can't have an exact copy of ourselves without that entity having conscious experiences.

Indeed, many might already claim that they can't conceive of a p-zombie. These thinkers feel they understand the physical world well enough to confidently say that the very idea of a p-zombie is illogical — it is just as impossible to imagine as a square circle.

This flips the zombie argument on its head, giving us what we might call the reverse conceivability argument. It goes something like this:

• Premise 1: Zombies are not conceivable.

• Premise 2: If zombies are not conceivable, then they are not metaphysically possible.

• Premise 3: If zombies are not metaphysically possible, then consciousness is physical.

  Conclusion: Consciousness is physical.

In response to this argument, Chalmers claims that any future scientific discovery, no matter how groundbreaking, will still be about the same old stuff — physical stuff. And he insists that no amount of this kind of knowledge could shake the core intuition that zombies are conceivable.

But this claim highlights a deeper issue.

Our ability to conceive (or not conceive) of zombies seems to be influenced by our starting assumptions. If we begin with the assumption that consciousness is non-physical, then imagining a p-zombie seems doable — you just picture a human but without the extra, non-physical consciousness bit added. But if we start with the assumption that consciousness is physical, the idea of a p-zombie seems nonsensical.

In other words, our assumptions about what we know shape what we think we can and can't imagine, and that affects whether we find zombies conceivable or not.

This is where the debate often reaches a stalemate.

So, who bears the burden of proof? Should proponents of zombies have to demonstrate that they are truly conceivable, or should their critics have to prove that they're not?

Challenging Premise 2: Conceivable Zombies Do Not Equal Possible Zombies

Even if we accept that zombies are conceivable, we may still wish to reject the zombie argument by challenging its second premise. We might agree that we can conceive of zombies without logical contradictions but dispute whether this imaginability guarantees possibility. This objection isn't just about zombies—it's a challenge to the conceivability argument itself.

The zombie argument is part of a family of arguments that all point to the same idea: there's a gap in our understanding between the physical world and our conscious experience. This is called an epistemic gap (epistemic referring to knowledge or the study of how we know things). These arguments suggest that even if we knew everything about the physical makeup of a person's brain, we still wouldn't be able to fully understand or predict their conscious experiences.

It's important to note that we're talking about a gap in knowledge here — a limit to what we can know or understand about consciousness based on physical facts alone.

But the zombie argument (and those like it) doesn't stop at pointing out a gap in our knowledge. They go a step further. They suggest this gap in understanding tells us something about reality. In other words, they claim that because we can't fully explain consciousness using physical facts alone, consciousness must be something more than just physical processes in the brain.

This line of reasoning takes us from a claim about the limits of our knowledge — an epistemological claim — to a claim about the nature of reality itself — an ontological claim (ontological refers to the study of what exists or the nature of existence). It's a bold and controversial leap. The question is, are we justified in making this leap? Or are we drawing too strong a conclusion from our limitations of knowledge?

To illustrate this point, let's consider a classic example often raised in philosophical discussions on conceivability — water as H₂O.

Let's imagine travelling back in time to an era when science hadn't discovered that water is H₂O. In this world, it's much easier to imagine water without oxygen. Fast forward to our time, and the same attempt to conceive of water without oxygen is more difficult. In our time, we know that oxygen is an essential component of the molecular structure of water.

In the pre-H₂O-knowledge world, even if we think we are conceiving of water without oxygen — our conceiving of it doesn't make it actually possible. We thought we were imagining a coherent scenario, but we were actually imagining something impossible—we just didn't know it at the time. Our epistemic limitation didn't reflect the ontological truth about water's composition. In other words, there was an epistemic gap but no ontological gap.

Just as we were mistaken about the possibility of water without oxygen, we might be mistaken about the possibility of physical duplicates without consciousness. The gap in our understanding—our epistemic limitation—doesn't necessarily imply an ontological truth about consciousness being non-physical.

Challenging Premise 3: Possible Zombies Do Not Imply Physicalism is False

Premise 3 is doing more work than a typical premise. It's a sub-conclusion that, when combined with the first two premises, establishes the argument's main conclusion.

Here's a more precise form of the argument, as presented in Chalmers' paper:

I’ll outline it, then we’ll unpack it.

First, he sets out some terms:

• P = the conjunction of all microphysical truths about the universe.

• Q = an arbitrary phenomenal truth about the universe.

• & = ‘and’

• ¬ = ‘not’

here’s the argument:

• Premise 1: It is conceivable that P&¬Q.

• Premise 2: If it is conceivable that P&¬Q, it is metaphysically possible that P&¬Q.

• Premise 3: If it is metaphysically possible that P&¬Q, then physicalism is false.

  Conclusion: Physicalism is false.

Let’s unpack. We’ll start with P&¬Q.

P represents everything that could be known about the physical universe. It includes all physical facts across all scales, from the quantum to the cosmic, including emergent phenomena like behaviour and societies. It's the complete physical description of the universe, leaving nothing physical out.

Q, on the other hand, represents what it’s like to experience something in that world, like the experience of the colour red or the feeling of happiness.

So, P&¬Q can be read as a world that is physically identical to ours (P) but without the feeling of what it is like to experience something in that world (Q).

Premise 1 claims that we can conceive of such a world.

But notice what is required for us to accept this premise.

Chalmers, in his paper, asks us to imagine a world that is physically identical to our world, down to the micro-level. This imagined world, however, lacks something. It's missing an element that exists in our world. The imagined world is devoid of Q.

We must conclude, therefore, that Q cannot be physical. If Q were physical, and the imagined world lacks Q, our world and the imagined world could not be physically identical. Q must be non-physical.

So we must define our terms as:

• P = everything physical in our world

• Q = non-physical things

Premise 1, therefore, can be read as — it is conceivable that there be a world that is physically identical to ours (P) but without non-physical things (Q).

Physicalists and non-physicalists would likely agree that such a world is conceivable — although, perhaps, for different reasons. Chalmers would call this possible world a zombie world. The physicalist would simply call it our world.

Let's turn to Premise 2 — if it is conceivable that there be a world that is physically identical to ours (P) without non-physical things (Q), it is metaphysically possible that there be a world that is physically identical to ours without non-physical things. Again, the physicalists and non-physicalists would agree.

It's Premise 3, where we encounter significant disagreement.

Premise 3 claims: If it is metaphysically possible that there be a world that is physically identical to ours without non-physical things, then physicalism is false.

From a physicalist perspective, this statement is a non sequitur — the conclusion simply doesn’t follow. Physicalists can easily conceive of a world that is physically identical to ours without non-physical things because they don't believe non-physical things exist in the first place. It would be like asking a physicalist to conceive of a world that is physically identical to ours but doesn't have Casper, the Friendly Ghost. Easily imaginable!

The concluding statement in Premise 3 comes as a surprise to the physicalist. If a physicalist is asked to conceive of a world that is exactly like our world in every way but doesn’t have non-physical things — why is physicalism false?

The problem with Chalmers' argument, it seems, is that it hides a hidden premise — (Q) is non-physical. By defining Q as something that can be absent while all physical facts (P) remain the same, Chalmers implicitly assumes the non-physical nature of consciousness from the outset.

The argument assumes what it sets out to prove. It assumes Q is non-physical and then claims that Q is non-physical.

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