Simulation hypothesis and substrate-independence of mental states

by Tiago de Vassal 5 min read23rd May 20185 comments


I’m grateful to Michaël Trazzi. We had a great discussion on this topic, and he then was kind enough to proofread the draft of this article.


Substrate-independence of mental states proves too much, so it is not necessarily true. The consequence is that simulations of conscious beings are not necessarily conscious themselves.

A simulation is only a means for an observer to see the development of a system over time, not the creation of that system. Hence, changing the simulation has no influence on the simulated system.

I - Introduction and definitions

The simulation hypothesis is not only widespread in philosophy, it has also found its way in pop-culture. The best known version of this idea may very well be Nick Bostrom’s “Are you living in a computer simulation?” (2003). My point along this article will be that it is impossible or unlikely for you to be in (as in “inside”) a computer simulation. To that end, I will provide some counterintuitive consequences of substrate-independence of mental states.

As stated in Nick Bostrom’s article, substrate-independence is :

“The idea is that mental states can supervene on any of a broad class of physical substrates. Provided a system implements the right sort of computational structures and processes, it can be associated with conscious experiences.”

Substrate-independence is a necessary condition for the computational theory of mind, wich is itself necessary for the simulation of consciousness as it appears in Nick Bostrom’s article.

Consciousness is the mental state associated with subjective, qualitative experiences (also named qualia).

II - What counts as a simulation?


When talking about simulations, we often picture a computer doing a huge amount of calculation, based on some chosen parameters describing a system accurately enough, and displaying the results onto a screen. For sure, that would be a simulation. But what happens if you remove the screen? Would it lead to the end of the simulation? What about the other parts of the computer, are they as expendable as the screen?

intuitively, simulations are closely related to computer simulations. But try to imagine the least technologically advanced simulation. What is it made of? In the history of electronics, we have seen computers made out of many different things. If you are willing to sacrifice calculation speed, you can use anything, from water flows to dominoes.

Picture a wealthy king in medieval Europe wanting to simulate a human brain. Electricity doesn’t exist yet, but he has access to virtually unlimited manpower. So instead of semiconductors and electricity, he arranges a network of men communicating with each other by sending written notes, each one with a very clear set of instructions. This peculiar computer is made of people. At the king’s command, they will compute by hand all the algorithms needed to run the rendering of a human brain. Give it sufficient time and men and you can do it. It will be excruciatingly slow, but it will work exactly as well as a modern computer.

Let’s now assume that the parameters fed into this primitive machine are close enough to the observed behaviour of a real brain. Do you think the simulated brain will experience consciousness the same way a real one would?

A close argument (The China Brain) was put forward by Ned Block in Troubles With Functionalism (1978).

The idea at the core of substrate-independence is functionalism (mental states are constituted by their function, in other words their causal role among other mental states). In this view, our simulated brain would be conscious.

The argument so far is that if a computer can produce a simulation of a conscious mind (as is stated in Nick Bostrom’s article), then a more primitive computer should also be able to simulate a conscious mind.


Let's begin another though experiment with the simulation of a conscious brain, displayed by a computer onto a screen that displays the outputs in real time. Assuming a conscious mind can be simulated, then this simulated brain would have conscious experiences.

Now, if you remove the screen, would the brain inside the simulation stop having consciousness? No, because the screen is only there for the observer to see the universe. If you remove the graphics card rendering the display of the outputs, would the brain inside the simulation stop having consciousness? No, like the screen, the graphics card has no role in the simulation process.

To what extent can you remove parts of the simulation, and the brain inside still being conscious? Does it depend on it being interpreted by an observer?

If you were to remove enough components, you would eventually end up being unable to observe the simulated brain. You would not receive any more information from the computer about the brain. But conscious experiences inside the simulation cannot emerge only from computation that makes sense to an observer. Otherwise, the definition of simulation would be loose enough and anything could be one. The only difference would be that some simulations are readable and others are not.

Yet another thought experiment : let’s say I use a pile of sand on wich I can blow at different angles and different speeds. Each blow counts as an input in some way, and each subsequent state of the pile of sand - the way the grains are spatially organized - is the output. Let’s say I have enough time, through trial and error, to compile and describe an isomorphic function from all the possible mental states of a conscious brain, to all the possible states of the pile of sand. Would you say that this pile of sand counts as a computer ?

If so, what happens if instead of me blowing on the pile of sand, it’s the wind (in the same way as I would) ? Would that still be the simulation of a conscious brain ?

To me, this is absurd. There must be something other than readability that defines what a simulation is . Otherwise, I could point to any sufficiently complex object and say : “this is a simulation of you”. If given sufficient time, I could come up with a reading grid of inputs and outputs that would predict your behaviour accurately.

If we extend the ability to simulate a conscious being to all forms of computation, along with the notion that computation depends on its interpretation by an observer, then many things can be said to be simulations of a conscious mind, wich is not intuitive.

III - The power of simulation - my interpretation

Some consequences of substrate-independence of mental states are absurd. Therefore, mental states are not substrate-independent, and conscious experiences cannot happen in simulations.

Simulation refers both to the process of simulating a system and to the system being simulated. From now on, to avoid confusion, I will call "rendering" the process of simulating, and simulation the system that is being simulated.

As I understand it, Nick Bostrom’s article presents a simulation as in the following diagram (with potentially other simulations inside universe B) :

Universe B is rendered by the computer in universe A. People in universe A can act upon (change their conscious experiences) people in Universe B, at least by shutting the computer down. This relation, as you might have understood, is represented by the red arrow and labelled “causal influence”.

I would say that rendering universe B does not entail the creation of universe B (in the sense of it or part of it being subject to conscious experiences). A simulation is like an analogy : it provides understanding, but changing the meaning of the source does not change the meaning of the target, instead it makes the analogy false. In the same way, acting upon the computer in universe A would only make the information it provides unrelated to universe B.

The simulation may be a way of gathering information about what is rendered, but it can't influence it. This is because the simulation does not create the universe that is being simulated. If you change the parameters of the simulation, the computer would stop giving you correct information, in other words, it would stop to predict accurately the behaviour of the system you were rendering.

I like to think of simulations like I do of symbols. Any scrawl can, in theory, mean anything to anyone. Another way to say it is that the meaning of a symbol is not a property of the symbol itself, but of the reader interpreting it. A simulation is nothing more than a proxy, or the outsourced understanding of a process. We rely of the predictable nature of a process to predict another process.

From this point of view, it follows that there is no ethical concerns to be had for the simulated mind. There is only the appearance of suffering. Would thinking about something bad be detrimental? No. Here, it is the same about simulation. The confusion emerges because there is much more predictive power in simulations.

IV - Other possible conclusions

I see two possible conclusions other than the one I presented in part III :

1. All computable minds exist, in the sense that they are or have been simulated to their fullest, and are able to experience subjective experiences

In other words, anything is a simulation of one or many conscious minds, even an infinite amount of conscious minds. This would be closely related to the ideas put forward by Max Tegmark in “The Mathematical Universe” (2007)

2. Mental states are only substrate-independent to some extent, and they can exist in some but not all simulations