Could you be living inside a simulation created by a more advanced intelligence? Where does your unerring belief that you are not come from?

Natalie Nicklin

The short answer is you don’t. Consider this: with every passing moment, we get closer to creating intelligent machines, maybe even conscious ones. If we can do this, could someone – or something – else do it too?

Philosopher Nick Bostrom at the University of Oxford highlighted this idea in 2003, arguing that if humans were one day able to create simulations populated with conscious beings, it’s at least possible that we, too, are living in such a simulation. Since then, that possibility has, if anything, become more realistic. There are projects seeking to build entire animal brains from scratch, modelled exactly on living ones, down to individual neurons and the myriad connections that interlink them. When very simple versions were given robotic bodies, lo and behold, they behaved like the creatures they were modelled on. It’s probably only a matter of time before we create virtual beings inside computers.

In all likelihood, we will never find out whether or not we are simulations ourselves. But one thing is clear, says philosopher Thomas Metzinger of the University of Mainz in Germany: each of us has a robust experience that “I exist”. Perhaps a slightly more manageable problem is to figure out where that experience comes from.

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Clues come from neuropsychological conditions such as Cotard’s syndrome, in which people are convinced that they do not exist. In 2013, Adam Zeman of the University of Exeter, UK, and colleagues reported their studies of a person with Cotard’s. His brain scans showed important anomalies. One was in a brain network normally associated with internal awareness, including the awareness of our body and its emotional state. Activity in this network was low, down to levels seen in people who are minimally conscious. The researchers speculated that this created a perception of non-existence which the man could not discount because other parts of his brain responsible for rational thought were also damaged.

The findings suggest that by creating a vivid perception of our body and its various states, our brain generates the feeling of existence – and any malfunction in this mechanism can cause us to question it.

How this all happens could be explained by the idea that the brain is a prediction machine. It is continually being assaulted by signals from the body and its environment and must predict what’s causing them. For example, when you are walking by the coast, the brain has to be able perceive that you are about to come to a cliff – if you don’t, you may fall off the edge. It does this by creating internal models of the body and the environment. To make accurate calculations, the brain must maintain prior knowledge and keep testing the integrity of its models. “The brain is a system that is continually trying to prove its own existence,” says Metzinger.

He thinks this prediction machinery might be compromised in people with Cotard’s. “The prediction error can never be cancelled out, [and this] attacks one of the most abstract and highest priors – ‘I exist’ – and makes that crumble away.” Of course, all of this certainty and doubting could still be part of a simulation. In which case, says Metzinger, “What I’d want to know is, what the heck is the hardware that the simulation is running on? Is it God’s brain or the Devil’s?”

This article appeared in print under the headline “How do I know I exist?”