Philosophical zombies sound like something from a meme, but philosophers have been thinking about zombies longer than most horror movie buffs and for good reason. They believe looking at zombies can help us determine the nature of the attributes constituting the human mind.

So, the first thing to know is that when philosophers use the term zombie they’re referring to philosophical zombies, or p-zombies, which are a hypothetical being constructed for the sake of a thought experiment. These zombies aren’t the walking dead; they’re instead like you and me except for one big difference — they don’t have consciousness or a mind. They have a physical brain, again like you or me, but even though the lights are on nobody’s home so to speak. To truly understand this strange idea, it’s important to first know the context behind the thought experiment and what ideas it emerged in response to.

Matter makes mind?

In philosophy, thought experiments are a means of obtaining insight on an issue. Thought experiments generally involve the creation of hypothetical concepts or even entire “possible worlds” where the implications of some particular idea can be fully explored. Zombies have been invoked as a response to certain proposed solutions surrounding a long contended metaphysical issue, the nature of subjectivity and the mind-body problem.

The full background of the mind-body problem is for another day, but the gist is that we as self-aware minds are unlike any other thing in the universe. Can our minds be described by the same properties that describe everything else that eixsts? Essentially, are our thoughts, feelings, emotions, and beliefs all reducible to inputs in a physical system? Although this question probably isn’t pondered very often by non-philosophers, it’s likely something that isn’t wholly foreign to the average person, and considerations like these might come up in discussions of spirituality and the afterlife. Contemporary philosophy doesn’t generally tread in these realms, but the question about the reducibility of the mind to a physical system could have large implications for lots of questions in philosophy like the problem of free will or our understanding of personal identity.

While there is yet no conclusive solution to the mind-body problem, contemporary science suggests that the universe and everything in it — including the human mind — can be reduced to matter reacting to physical inputs. At the very least this point of view, often called materialism or physicalism, is a necessary operating assumption critical to the progression of science. Philosophers who pose the zombie problem are going against the grain by questioning the completeness of physicalism. They usually don’t deny that physicalism is at least partly right, but for them, it isn’t the whole picture as there’s something else accounting for our minds. These philosophers are usually called dualists because they propose that reality consists of two aspects, a physical aspect and a second one. It’s important to know that this “second aspect” differs from philosopher to philosopher, even among those who support or are sympathetic to the philosophical zombie argument. In fact, it’s not necessarily the goal of the p-zombie thought experiment to show us what’s missing but to simply indicate that something is indeed missing.

As an aside, it’s sometimes common for non-philosophers to assume dualists who use thought experiments like the philosophical zombie problem are arguing for the consideration of something like a soul. However, many contemporary dualists are naturalists who believe that all aspects of the world, even secondary aspects, come from nature. Although a zombie argument can be constructed for souls, in its most popular forms the p-zombie thought experiments aren’t used this way.

The anatomy of a philosophical zombie

So what exactly makes zombies tick? The simplest proposals suggest that zombies are atom-to-atom the same as any other person. What does this mean? For the sake of example let’s say we’ve met two young women named Eliza with one being a zombie counterpart of the other. Because they’re the same on an atomic level they look the same, but we know that one is a zombie and we have to figure out which one is the zombie. How should we go about figuring it out?

Well, any tests of sensation or reflex will likely show us nothing, as both human anatomy and zombie anatomy are exactly the same. For example, pinching a zombie will cause an electrochemical response that would probably make it say “Ouch!” The zombie, of course, isn’t aware that it’s a zombie because it doesn’t know what it doesn’t have. If you asked zombie Eliza why she said “Ouch!” after pinching her she’d say, “Because it hurt!” But zombie Eliza is only functionally similar to Eliza, and her physical systems (brain included) react to inputs the same way real Eliza’s does; however, there is no subjective notion of self making sense of zombie Eliza’s experiences. Zombie Eliza can be thought of as a flesh-robot who can imitate Eliza’s behavior without experiencing the sensations integral to feeling what it’s like to be Eliza. This aspect of “what-it’s-like-to-be-ness” is sometimes referred to as qualia in philosophy, and the question of how to define these types of experiences has been key to many thought experiments questioning whether or not atoms (or physical properties in general) can of themselves explain our notions of qualia.

As far as our own efforts to tell both Elizas apart, unless we can see into the mind of both Elizas to project and compare their personal, subjective experience we’re at an impasse. To those who don’t think that matter is all there is to the story, this is sufficient, but to zombie argument opponents this impasse highlights one of the many problems with this thought experiment.

What if p-zombies are overrated?

Just like with pop culture zombies there are a lot of people who question the appeal of philosophical zombies. There are several types of arguments that have been proposed which attack the logic, feasibility or implications of zombie-based arguments and thought experiments.

We are p-zombies/how do we know we aren’t p-zombies?

Certain materialists might, for the sake of argument, acknowledge that according to the definition given by p-zombie proponents, we are philosophical zombies. To these opponents, though, this just proves the absurdity and uselessness of the concept. But even materialists who don’t take the argument against p-zombies this far contend that since we can’t tell apart zombies from non-zombies, in a world where these entities existed, we’d have no way of knowing that we weren’t philosophical zombies. Given that zombies are physically similar to humans, a zombie clone asked what it feels would return the same response that its non-zombie counterpart would give. If it looks like a duck and walks like a duck, does it matter if it supposedly lacks something we can’t measure? We can’t measure whatever it is zombie proponents are arguing for within ourselves, and we aren’t zombies (as far as we know 😉). Effectively, either the zombie test is terrible or the argument for qualia — or soul, or whatever it is that’s missing — implied by the zombie thought experiment is too weak to be useful.

Conceivability as a criterion

Proponents of philosophical zombies aren’t arguing that zombies are real or that they could even exist in our universe (though perhaps there’s some awesome zombie multiverse out there), they’re claiming that since we can conceive of the idea of a zombie, it entails that physicalism isn’t sufficient in explaining qualia. There are a variety of arguments attacking the notion that conceivability is a useful criterion, however. One popular argument is that even if the idea of a zombie were conceivable and one could exist, that tells us nothing about the state of the world we currently live in. In other words, conceivability and possibility aren’t linked because merely conceiving of something cannot tell us the mechanisms by which it is possible and whether or not those mechanisms exist in our universe.

Impossibility and/or incoherence of philosophical zombies

A very popular critique is that philosophical zombies, as defined by proponents, are inconceivable because their attributes contradict one another in the same way that something like a ‘four-sided triangle’ contradicts itself. Daniel Dennett, one of the most outspoken critics of p-zombie arguments suggests that ideas like consciousness are not constructs referring to a single thing, but to a collection of things forming a system of processes. This means viewing consciousness as a single aspect that can be added or taken away from a body is incorrect. Dennett uses the concept of health to illustrate his point. In The Unimagined Preposterousness of Zombies he writes:

To see the fallacy, consider the parallel question about what the adaptive advantage of health is. Consider “health inessentialism”: for any bodily activity b, performed in any domain d, even if we need to be healthy to engage in it (e.g., pole vaulting, swimming the English Channel, climbing Mount Everest), it could in principle be engaged in by something that wasn’t healthy at all. So what is health for? Such a mystery! But the mystery would arise only for someone who made the mistake of supposing that health was some additional thing that could be added or subtracted to the proper workings of all the parts. In the case of health we are not apt to make such a simple mistake, but there is a tradition of supposing just this in the case of consciousness. Supposing that by an act of stipulative imagination you can remove consciousness while leaving all cognitive systems intact–a quite standard but entirely bogus feat of imagination–is like supposing that by an act of stipulative imagination, you can remove health while leaving all bodily functions and powers intact. If you think you can imagine this, it’s only because you are confusedly imagining some health-module that might or might not be present in a body. Health isn’t that sort of thing, and neither is consciousness.

Many materialists reject p-zombie arguments on similar grounds as they argue that consciousness is emergent. When people talk about emergent properties and emergent phenomena, they’re essentially saying that a system is greater than just the sum of its parts. If consciousness is indeed emergent, then the properties of individual atoms or even collections of molecules alone won’t help you understand consciousness; you’d have to understand each atom in relation to others within the system in which they’re interacting. Since science has yet to reach this point, for proponents of emergence, it makes sense that consciousness still seems mysterious to us despite our understanding of physics in other contexts. They argue, though, that this mysteriousness need not be explained by something even more complicated. In this regard, an emergent materialist might conclude, like Dennett, that philosophical zombies are incoherent since consciousness emerges from our composition through undiscovered emergent properties.

Why don’t philosophical zombies just die?

The problem of philosophical zombies is very old, going back at least to Descartes’ understanding of the mind-body problem. It’s clear that this problem tries to get at what makes us fundamentally human, metaphysically speaking. It’s also relevant to how we should interpret the world and has implications for how we relate to not just human minds, but other ones too. If consciousness is emergent, does that mean that other physical systems have varying degrees of consciousness that I just can’t perceive? We probably intuitively understand this is the case for animals like our pets, but if dualists are wrong, then it’s possible that even the artificial intelligence systems we’ll build one day could develop emergent consciousness. The thought experiment also touches on broader ideas too. Is materialism/physicalism just an assumption we make for a coherent science or is it really true? This problem surrounding how to understand the properties of physical systems will be seen in a lot of other thought experiments that we’ll talk about in the future.