I think, therefore I am. But there are those that suggest you are not who you think you are.

We depend on our senses to perceive the world. We understand it through our eyes, and ears, through touch and smell. But should we question what our senses tell us? Should we consider that what we touch, see and hear may not be real?

In 2016, renowned physicist Neil deGrasse Tyson put the odds at 50-50 that we are not real flesh and blood beings but are in fact software simulations, and that our entire existence is just a program in someone’s computer. Entrepreneur and Tesla boss, Elon Musk, has famously said he also believes it more than likely that we live in a simulation.

And, frankly, as we shall see, it is difficult to argue against the idea.

But then there have always been those who doubt the evidence of their own eyes, their senses, their brains. From an evil demon imagined by Descartes, through a mad scientist’s Brain in a Vat and celestial Boltzmann Brains, to Simulation Theory, people have always suspected that, what we see as reality, is actually an illusion.

René Descartes, the 17th century philosopher, considered a scenario where he was subject to the influence of an evil demon. "I shall think that the sky, the air, the earth, colours, shapes, sounds and all external things are merely the delusions of dreams which [the demon] has devised to ensnare my judgement. I shall consider myself as not having hands or eyes, or flesh, or blood or senses, but as falsely believing that I have all these things."

In other words, everything that Descartes senses is an illusion created by the demon.

Whether the evil demon really exists or not is not important, Descartes is giving us grounds to doubt everything; especially material things.  The consequence of this thinking is that we cannot trust our senses: that the reality that we perceive may be an illusion invented by some powerful, or supreme being. But, we can have knowledge of at least one thing: we exist.

If, he suggests, there is "a deceiver of supreme power and cunning who is deliberately and constantly deceiving me" then Descartes himself clearly exists. The deceiver, he says, can "never bring it about that I am nothing so long as I think that I am something". This is, of course, a version of Descartes famous philosophical statement “cogito, ergo sum” or “I think, therefore, I am”.

If one doubts the existence of reality, then one is thinking—and for thinking to happen, there must be someone doing that thinking.

The controlling being does not have to be a demon or supreme power. Consider the situation where a mad scientist has removed someone's brain and is keeping it alive in a container, and immersed it in some sort of sustaining medium. The scientist has connected the brain to a supercomputer which simulates the human senses. The brain cannot tell the difference between the real world and the one being simulated by the mad scientist. This is the Brain in a Vat scenario.

In a slightly different version, James Cornman and Keith Lehrer suggested something called a braino machine. It consists of a special cap worn by the subject, and when placed on the head, "operates by influencing the brain". When the braino cap is placed on a subject's head, the operator can produce any hallucination in the subject’s brain. Again, the subject cannot tell the difference between the braino cap-induced hallucinations and reality.

The outcome of the Brain in a Vat situation is not so dissimilar to that created by Descartes’ demonic deceiver. The brain does exist and is conscious, but the environment that it perceives is entirely false—it is simulated by the mad scientist’s computer or the braino cap.

And then there are disembodied brains that are randomly created, floating in a vast sea of subatomic particles, complete with the illusion of outside stimuli and realistic memories.

Ludwig Boltzmann was a 19th century physicist and philosopher, the time when the developing ideas about thermodynamics culminated in the formulation of the famous three laws of thermodynamics. The Second Law of Thermodynamics states how all closed systems become more disordered over time and, consequently, the Universe will end up as a completely disordered mess.

We see the transition from order to disorder everywhere in life: matter decays, iron rusts, rocks crack and crumble. As an example, imagine a small child building a tower of wooden blocks. She makes a perfect tower, sides straight and the blocks perfectly lined up. She is so proud of her achievement that she wants to show her father, so she picks up the tower but stumbles and drops them. No more neat tower, just a disordered pile of blocks. But no matter how many times you gather up the blocks and drop them again, they will not reform the tower. As the Second Law tells us, we never see order spontaneously come from disorder.

Boltzmann took a probabilistic view of this Second Law. The disorder that emerges results from this being the most probable state for a system to be in. There are many more possible disordered states than ordered ones so, mostly, the disordered one wins. So in his view, the reformation of the block tower is not impossible, just extremely unlikely.

According to Boltzmann, while improbable states are unlikely to occur, they are not impossible.

Though Boltzmann didn’t consider the idea of ‘Boltzmann brains’, they are an inevitable consequence of his theories. The future dead universe would contain all of the matter that exists, but through entropy it would have been transformed into a warm soup of elementary particles, randomly dispersed throughout the Universe. According to Boltzmann, these particles could randomly come together to form—well, anything. Including a brain complete with past thoughts and memories. But then,in the same way that they popped into existence, they disappear again.

That’s highly improbable, of course, but in the infinity of time even the ridiculously improbable will happen eventually.

Improbable may seem a mild word for it, but the existence of Boltzmann brains is consistent with current scientific thinking (not that the theoreticians like the idea much). And if they can exist, in the infinity of time, many more of them will be created than there are human beings. So, statistically, you are more likely to be a Boltzmann brain than a real human!

You might think that Descartes or the brain in a vat find themselves in unfortunate circumstances, but as a Boltzmann brain your situation is worse. Because in your fleeting existence, you do not think - therefore you aren’t.

Simulation Theory is a somewhat different and technological theory, but based on thinking that is similar to these other ideas. Unlike our previous examples, Simulation Theory has, perhaps, the benefit of being a little more plausible.

Oxford University philosopher, Nick Bostrum, wrote about the theory in 2003. The basic idea is that we are not natural beings, but characters in a highly sophisticated computer simulation.

However, Bostrum's argument relies on a couple of assumptions. The first and most fundamental one is that the functionality of human brains does not rely on carbon-based biology — the grey matter inside our heads — but could be replicated in electronic circuits and software. He further assumes that we can continue the enormous progress we have already made in creating life-like computer simulations, until we get to the point where we can simulate an entire human brain—lots of them, in fact. This, then becomes the basis of the simulation of real beings like you and me.

Bostrum considered three mutually exclusive propositions. The first is that civilisation will end before we get to a level of development where we can create these complex simulations. The second possibility is that we figure out how to create these simulations, but decide not to build them—perhaps because we believe it is unethical to simulate humans.

And then, there’s the third possibility: we’re living in a simulation.

If we are to believe that the last proposition is true then, clearly, we have to also assume the first two propositions are false. We must assume that civilisation will continue to develop, and doesn’t destroy itself in a nuclear holocaust or some other self-destructive activity, and that our increasingly refined computer simulations continue to become more and more complex and life-like. We must also assume that, in the future, we will have no moral or ethical objection to making these highly sophisticated and life-like simulations.

This leaves us with the situation where our future selves will have the technology available to create simulations that incorporate simulated brains, and that they will do so.

These simulations could be of us, of our ancestors, or purely fictional, like vastly more sophisticated versions of the simulation games of today. They would be real to the simulated beings within it—indeed, for them it would be life. A real virtual world. There could be millions, or billions, of virtual people having virtual partners and families, working at virtual jobs in virtual companies, belonging to virtual nations which have virtual wars. To the inhabitants of the virtual world it would be completely authentic.

But since the simulation is modelled on reality, among the virtual people there will be virtual software developers and virtual entrepreneurs. And, these virtual software engineers will create simulations.

Simulated beings will begin to develop simulations, too. There will be virtual worlds within the virtual worlds, and—it follows—virtual worlds inside them, too. We end up with a Russian doll of virtual worlds, and there is no limit to the levels of doll inside.

Consequently, there are many more simulated beings than real ones, so which is more probable: that we are the original and real civilization at the top of the pyramid who will one day go on to create these simulations, or that we are actually one of them? Statistically speaking, it's much more likely that we are living in a simulation.

But does it matter? Does being real or being simulated make any difference to us? Probably in our day-to-day life it doesn't make any difference. But if we are living in a simulation, then it is also probable that the universe that we think we are seeing is just an illusion.

To demonstrate why this is the case, we need to think about why future civilizations would make these astonishingly sophisticated simulations.

Simulations can be used to predict future events, to demonstrate theories or for entertainment. But it does not need to be complete. Even if it were possible to create a simulation of the entire universe — us included — it would take a great deal of effort and there would be no benefit as we (the simulated beings) cannot possibly interact with it in any meaningful way; we can only observe it.

If you want to create a simulation of an entire planet and of all the people on it, you would also need to simulate the nearby environment. So if a simulated humanity progresses enough to create astronauts and space travel, then the simulation would need to include the moon and other planets. But anything beyond the reach of our simulated society need not be included in the simulation, it would only need to look right from the point of view of the simulated Earth.

So, the likelihood is that, if we are living in a simulation, the rest of the universe is an illusion. Like the backdrop in an old episode of Star Trek, it’s not really there.

An earlier version of this article appeared in the Apeiron Blog on Medium.com.

Curious for more? Sources and references for this article can be found here.