Reality II: Bostrom

Review: Computation

• Computation is defined as a transition from input and starting state to
output and end state, mediated by representations.
• Input and Output = Different signals sent (respectively) to, and from, a system

• Simple I/O Device = Light Switch

• State = The way something is at a time
• State transition = Change in state (e.g. going from hungry to full)
• I + SS → O +ES Device = (older) Coke Machine

• Representations = A physical object that is about something else
• Object = Vehicle
• What it is about = Content
• Example: Patterns of pixels, ink, and sound waves are all physical objects that can be about

• In a newer Coke Machine, the transition from input to output, and from starting state to end

state, is mediated by (pictoral) representations of the input.

Review: Computational Theory of Mind

• Claim that the mind is essentially computational
• Compatible with Physicalism/Identity Theory

• In that case, claims that mind is realized in or implemented by brain

• But also compatible with dualism
• Mind is implemented by immaterial substance

• Think of a person’s perception (stimulus) as input, behavior (response) as output
• The stimulus is represented and combined with the mental states active in a person at a time, such as beliefs and desires.

• That combination of perception, beliefs, and desires, leads to a behavior and a possible state change in beliefs and desires

• E.g. [See a sandwich → Represent [SANDWICH], Belief [THERE IS A SANDWICH], Desire [SANDWICH] → Behavior of eating Sandwich, De-activation of Desire

• Two minds can be computationally continuous
• Stricter than—and entails—psychological continuity
• Entailed by brain identity/continuity (if CTM is true), but does not entail it
• Sufficient for persistence (numerical identity?)

• Support for no: Mindscan (upload mind to computer) involves computational continuity but not persistence

• Support for yes: Augmentation (gradual replacement of brain by cybernetics) plausibly counts as persistence

• Two minds can also be computationally equivalent in complexity
• Possibly relevant for personhood!

• Explains how Dennett/Brain in Vat are possible
• Dennett = input signals travel over longer distance
• BIV = input signals that are normally generated by sense organs are generated by external computer

Key Concepts

• Sim World = a world that is entirely run on a computer(s) and populated by sims
• Sim = a being in the sim world

• Virtual Reality = a world that is entirely run on computers but inhabited by people in the physical

• Nested World = A world inside another world

• Base World = World containing another world
• Can be relative (World 1 is the base world of World 2)
• Or absolute (This world is the base world = not nested in any other worlds)

• Phenomenal Aspect of World = The sum total of sensible objects properties of a world (think
phenomenology = experience, phenomenon = object of experience)

• Substrate of World = The most basic level of a world, what underlies the phenomenal aspect
• Sim worlds/Virtual realities have code as their substrates
• Substrate of a Mind = that which a mind is realized on

• Intuitively, our minds are realized in brains
• But the mind of a Sim can be realized in the same code as the substrate of a sim world
• It can nonetheless be computationally equivalent to (as complex as) our minds!


• What do we mean by ‘reality’?
• In one sense, the real world is whatever world we inhabit.
• But we generally think it means more than that.

• Our intuitive picture of reality
• Reality is mind-independent
• The substrate of reality is matter and energy
• Reality is the (absolute) base world

• Skeptical scenarios challenge these assumptions
• Descartes’s Evil Demon challenged the first one

• The world was not independent of the mind of the evil demon

• Bostrom will challenge the second and third
• When we are presented with a challenge to any of the premises of the ‘intuitive picture’ we have a

• Concede that the world we are in is not the real world
• Or re-define reality
• Chalmers will discuss this question.

Nick Bostrom

• Philosopher at University of Oxford, UK

• Interested in intersection of philosophy and

Bostrom’s Thesis

• Bostrom makes three claims:
1. It is possible that we are Sims in a Sim Program.

2. It is possible that we are Sims in a Sim Program and do not know it.

3. It is overwhelmingly likely that we are Sims in a Sim Program and do not
know it.

• Let’s cover each in turn.

Is it Possible We are Sims?

• Bostrom says “Yes!”

• Bostrom is definitely right
• One key point is:

• Given sufficiently advanced technology, a Sim World can be phenomenally equivalent to
a non-Simulated World

• It would only differ in substrate

• The other key point:
• According to Computational Theory of Mind, our mind is implemented in the brain, but

what makes it a mind is the information it processes.
• Given sufficiently advanced technology, and If CTM is true, minds just like

(computationally equivalent to) ours can be realized in brains or computer programs
• If personhood depends on a certain level of computational complexity (up to and

including computational equivalence to a human mind) then Sims can be persons.

Is it Possible We are Sims and Don’t Know It?

• Bostrom says: Yes!
• Again, I have to agree with Bostrom.

• The difference between a sim world and our intuitive conception of a
real world is entirely about the substrate (it is code for a sim world)

• The difference between a sim and our intuitive conception of
ourselves is entirely about the substrate (it is code for a sim)

• But we cannot perceive substrates!
• We can only perceive the phenomenal aspects of our (a) world

• We cannot empirically determine the substrate of the world we are in!

• Compare to Descartes, especially the emphasis on sense perception

Is It Likely We Are Sims?

• Again, Bostrom says “Yes!”
• Note that this point does not follow from the possibility point. One could think it is

possible but exceedingly unlikely that we are sims.

• Bostrom makes the following argument.
• He first gives three statements; let’s call them A, B, and C.

A. The chances that a species at our current level of development can avoid going extinct
before becoming technologically mature is negligibly small.

B. Almost no technologically mature civilizations are interested in running computer
simulations of minds like ours.

C. You are almost certainly a simulation (Bostrom, 23).
• He then gives the following logical argument:

1. One and only one of [A, B, C] is true.
2. A is false.
3. B is false.
4. Therefore (From 1,2,3) C is true.

Bostrom’s Argument

• A is false.
• A = The chances that a species at our current level of development can avoid going

extinct before becoming technologically mature is negligibly small.
• Bostrom thinks that A is only true if:

1. Nearly every sentient species, when it becomes sufficiently advanced, develops some
technology T.

2. Developing technology T guarantees (or close to it) the extinction of the species (e.g.
nuclear weapons, something that destroys environment).

• B is false.
• B = Almost no technologically mature civilizations are interested in running computer

simulations of minds like ours.
• Bostrom thinks B is only true if no society has any interest in running a program with

Sims that are as sophisticated as their technology allowed.
• But we already do this!

The Sims
• The game “The Sims” is a

program that runs
simulated minds at the
highest level our
technology allows.

Bostrom’s Argument

• If A and B are false, then C must be true.

• C = You are almost certainly in a simulation.

• Let’s grant that A and B are false.
• Why must C be true?
• Because the only reasons we wouldn’t get to the point where we can have Sims as mentally complex as we

• We either die before we get there, or
• Don’t care once we do
• These are just A and B

• Ok, fine. But why can’t we just possibly be in a simulation?
• Bostrom’s answer: Sims are cheap. Unless a society decides that there can be some upper limit of sims, then

there is no reason that each person in a society cannot have a program with millions of Sims. Think about our
Sim programs!

• Suppose a society has 10 billion people, and has technology that allows for a program with 1 million sims
(arbitrary number).

• Then, assuming one program per person, that society has 10 quadrillion sims.
• You do not know if you are a Sim or not.
• On that planet alone, it is therefore 1 million times more likely that you are a Sim than a real being.

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