In his Meditations on First Philosophy, René Descartes famously described how he could doubt the existence of a world beyond his own mind. He thought he was sitting by the fire in his dressing gown writing, but could he be absolutely certain that he wasn’t in bed dreaming? He couldn’t. He couldn’t be absolutely certain that he had eyes or hands or a body, or that there was a physical world at all. The only thing he could not possibly doubt, he concluded, was that he was thinking – that certain events were occurring in his mind. This isn’t implausible. Consider your current situation. You think you are reading the words on a page in front of you. But can you be absolutely certain that there is a page there? Can you completely rule out the possibility that you are dreaming or hallucinating or, more outlandishly, hooked up to a simulated world, like the characters in the movie The Matrix? It’s not obvious that you can. But you can be absolutely certain that there appears to be a page in front of you – that you are having experiences as of a page. Even if you are really dreaming or in a simulation, you’re still having these experiences. How could you possibly doubt that?

There is a conclusion often drawn from this. It is that we are not directly aware of the world around us but only of appearances of it – mental images of colours, shapes, sounds, smells, and so on, created by our sense organs. These images (sense data, as philosophers call them) are the same whether we’re perceiving a thing or hallucinating it, and we can’t be mistaken about them. It is as if there is a theatre inside our heads displaying a movie of the outside world. Daniel Dennett, in his book Consciousness Explained, calls it the Cartesian Theatre. The show in this theatre is our immediate reality, and we infer the existence of the physical world, including our own bodies, from what is presented there. We are isolated beings, sealed off from the world and each other behind a veil of perception.

This view, in which the external world is real but known to us only indirectly, is called indirect realism. In the past, the view was usually coupled with a view about the mind itself, the spectator in the mental theatre. Descartes and many other indirect realists took this to be an immaterial soul. They thought that the brain receives signals from the sense organs and relays them to the soul, which does the real thinking and decision-making, before sending its commands back to the brain for execution.

Nowadays, few philosophers believe in an immaterial soul, and most think that minds are just functioning, embodied brains. And with no soul, the notion of a Cartesian Theatre looks less attractive. What would the spectator be? It would have to be a boss system within the brain itself. But it is hard to see why ancillary brain systems would need to display information to their boss in the form of images. The language of the brain is neuron firing, and once the sense organs have transduced stimuli into patterns of neuron firing there is no need to convert these patterns back into a sensory medium. As Dennett puts it, there is no second transduction. Besides, even if there were, the boss system would need to reconvert the images back into neuron firings in order to do any work with them.

As a consequence, indirect realism and sense data have dropped out of fashion. It’s more plausible to think that we perceive the world directly. This doesn’t mean that the mind magically reaches out to things. Perception depends on sensory processing systems in our brains, which track features of the world and pass information about them on to control systems. But we don’t perceive these processes themselves. We are not inner spectators or boss brain systems but whole organisms, and we are aware of something when sensory information about it reaches a wide range of control systems within us, enabling us to think about it, talk about it, remember it, and react to it in flexible ways. Processes in our brains make us aware of the world, but we don’t need to be aware of them in order to be aware of the world.

So, have philosophers now rejected belief in a Cartesian Theatre? Not wholly. The old view still keeps a hold over many of them, and reflections like Descartes’s still play a part in it. Think about hallucination again. Suppose that, like Macbeth in Shakespeare’s play, you seem to see a dagger floating before your eyes. Now if there really is a dagger there, then that’s what you’re aware of. But what if there isn’t one? You’re still aware of something, aren’t you? And since it’s not a physical dagger, it must be a mental one (a “dagger of the mind”, as Macbeth put it), painted with mental colours. So, it seems, there must be mental images after all. But if direct realism is true, then these images can’t be sense data – things we perceive. What are they then? A common answer is that they are mental properties that accompany our perception of things and make it conscious. They give perception a subjective quality – make it like something – and they are commonly referred to as qualia or phenomenal properties.

Put simply, the idea is this. Sensory systems in our brains gather information about the world and make it available to control systems. This is perception, and in itself it is not conscious. We can imagine creatures whose sensory systems work just like ours but who are not conscious. In us, however, perception is usually accompanied by a private qualia show, and this is what makes it conscious. When we hallucinate, the qualia show occurs without our actually perceiving anything.

This view is an attempt to hold onto our Cartesian intuitions about mental reality while endorsing a materialist view of the mind. It does this by side-lining mental images. It strips them of their old role in perception (qualia are, in effect, unemployed sense data), and turns the Cartesian Theatre into a Cartesian Sideshow, whose only function is to give experience a subjective quality. Despite these changes, however, the view still faces huge problems. How is the sideshow created, where in the brain does it happen, why is there a second transduction, and who or what is observing the sideshow? Some philosophers speculate that qualia are non-physical properties, which arise spontaneously in suitably complex brains, but this doesn’t really shed much light on the issue. At any rate, if we think that the mind is a purely physical system, then it looks as if we shall have to go the whole hog and dispense with the idea of a Cartesian Sideshow as well. But what then do we say about hallucinations and suchlike? If there aren’t any qualia, what is it that we are aware of when we are hallucinating?

Let’s go back a little. Our starting point was that there is something we can be sure of: how things seem to us. You can’t be sure that there is a dagger, but you can be sure that there seems to be one. We need to examine this a bit. First, can you really be sure how things seem to you? Right now, while you are looking at this page, what do you seem to see at the far left of your visual field? (Don’t move your eyes or your head; just say what you seem to see out there at the side.) You may be surprised. We tend to assume that our visual field is rich and detailed right out to the edges, but when we try to say what it is like, we find it hard to do so. Perhaps we are not so sure of how things seem as we think. But, you may say, we can at least be sure of how things seem in the centre of our visual field, where we are focusing our attention. Let’s grant this. The question is whether it follows that there is a Cartesian Sideshow.

The case for thinking that there is relies on a rather subtle move. We start with the claim that it seems as if there is something out there – a dagger, in our example. And we move from this to the claim that there really is something in here – a mental dagger, an appearance of a dagger. This move may look innocuous: there’s something dagger-like, and if it isn’t a physical dagger, then it must be a mental one. But we don’t have to make it. We don’t have to understand, “There appears to be a dagger” as meaning “There is a dagger appearance”.

How else can we understand it? Well, we might say that it means that we are disposed to think and react as if there were a dagger – a dagger with a certain precise shape, size, pattern, and colour. Think about what is actually happening. Our perceptual systems are being activated in the same way as when we see a dagger. Information about the presence of a dagger is being passed to control systems and affecting our dispositions in countless ways, creating beliefs, evoking emotions, triggering associations, and prompting us to perform various actions. This is what seeming to see a dagger is – having a host of dagger-related cognitive, emotional, and behavioural dispositions.

You may object that when we are hallucinating something we don’t always think and react as if the thing is real. We may suspect that we are hallucinating and not react at all. Yet this doesn’t change the way things seem. This is true. We can exert higher-level control to override the dispositions generated by our perceptual systems. But the dispositions will still be there, exerting a pull on our thoughts and actions. Imagine you’re looking at an optical illusion where a static spiral seems to rotate and expand. Even if you know it’s an illusion, you still can’t shake off the sense that there is movement, perhaps of a startling kind. Similarly, when you are hallucinating a dagger, your visual system keeps shouting, “Dagger there! React!”, even if you don’t believe your eyes. You can ignore the message and override the commands, but you can’t suppress them altogether. It’s a bit like accidentally triggering a burglar alarm. You know there’s no burglar, but you can’t shut the alarm off. It’s not surprising that our perceptual systems have this automatic, inflexible character; they are basic survival systems, tuned up to track things we need to know about.

Still, you may say, this doesn’t account for the reality of experience. Experiences have a quality to them – what it’s like to see yellow, hear a violin, taste salt, and so on. Surely, this can’t just be a matter of tracking and reacting to features of the world? After all, there are no such qualities in the world. Science tells us that colours are reflective properties of surfaces, sounds pressure waves in the air, tastes chemical compounds in food, and so on. Experiencing these things (the objection goes) must involve mental qualities – qualia. So, again, there must be a Cartesian show.

As before, we can concede some of this. When we ask ourselves what our experiences are like, we are attending to something about ourselves. But this needn’t be a qualia show. Maybe what we are attending to is just the complex of effects the thing we’re perceiving is having on us – the set of cognitive, emotional, and motivational dispositions already mentioned. That’s what the experience is. Here’s the general idea. The brain monitors its reactive processes and generates a simplified, schematic self-model, which captures the overall shape of the reaction pattern being activated. Different things evoke different types of reaction patterns (scary things one type, cute things another, and so on), so this self-model carries information about the significance of what we’re perceiving – our personal take on it. This information can then be used for the purposes of communication and self-control, enabling us to tell others about our experiences (the dagger was frightening and sinister), deliberately seek out experiences we like, and take steps to avoid ones we don’t. Now, when we attend to our experiences (when we introspect), we access information from this model. The information is actually about our reactive dispositions, but since the model is a simplified, schematic one, we can’t describe these dispositions in any detail. All we can say is our experiences have a distinctive shape or “feel” to them, which we find pleasant, disturbing, enticing, or whatever. It is this that leads us to believe in mental qualities, qualia.

Now, whether you are perceiving something or only hallucinating it, your self-model will be the same, since the same reactive dispositions will be generated. And these dispositions are perfectly real, just as an accidentally tripped burglar alarm really is sounding. So, there is something of which you can be more sure than you are of the world around you. But it is not an internal mental image of that world. Rather it is your take on it – the set of reactions it is evoking in you, as registered by your self-modelling system. And, unlike a Cartesian show, this is not essentially private. It is itself a part of the public world, a complex dispositional state of your brain-body system, which other people could know as well or even better than you do.

Of course, your take on the public world may not be correct. We can get specific things wrong, as when we hallucinate or are the victims of an illusion. And we can’t completely rule out the possibility that we’ve got everything wrong. Maybe we really are in the Matrix. But it doesn’t follow that there is another, private world, on which our take is always correct. Indeed, even if there were such a private world, we’d still need to track what was happening there (what was playing in the Cartesian Sideshow), and we could still be wrong about that. All we’d really know is our take on that world. Moreover, we couldn’t be absolutely sure even about that, since our self-model could be inaccurate. In the end, we have no absolutely secure awareness of anything, just attempts to track and interpret the world, including that part of it that is ourselves.

Four centuries after Descartes sat reflecting by the fire, we are still seduced by the idea that our immediate reality is a mental one. But it is a misguided and unattractive idea. The Cartesian Sideshow is a lonely place, where the show is private and there is only one spectator. Cartesian beings can never fully know each other or share their experiences. Thankfully, there is good reason to believe that we’re not such beings. Our reality is a shared public world, and while we each have our own personal take on this world, this isn’t essentially private and can be known by anyone who studies us carefully enough. We don’t need a Cartesian Sideshow. We have the world and all that it means to us.

Keith Frankish is honorary reader in philosophy at the University of Sheffield. Details of his work and publications can be found on his website, www.keithfrankish.com.