Neuron Replacement
Neuron Replacement
Also known as: Fading Qualia, the Silicon Chip Replacement, gradual replacement argument
Overview
Suppose one neuron in your brain is replaced by a silicon device that takes exactly the same inputs and produces exactly the same outputs with the same timing. Nothing about your behaviour changes, by construction, since every other neuron receives what it would have received. Now replace a second, a third, and so on, until the brain is entirely silicon. At every stage the person reports, sincerely and in detail, the same vivid experience as before.
Purpose
To test whether consciousness depends on the material of the brain or on its functional organisation. The argument was given its modern form by David Chalmers in "Absent Qualia, Fading Qualia, Dancing Qualia" (1995), building on an earlier version by Zenon Pylyshyn (1980).
Key Question
If consciousness depends on the substrate, what happens to it during the replacement? There are only three options: it is unchanged throughout; it fades gradually while the person continues to report full experience, so that they become radically mistaken about their own mind with no functional trace of the mistake; or it switches off at one particular neuron, which makes that neuron the seat of consciousness. The second and third are absurd, so the first is left, and consciousness is organisationally invariant.
Example
Chalmers' "dancing qualia" variant installs a switch that flips a region of the brain between its biological and silicon versions. If the two differed in experience, the subject's experience would change back and forth with the switch while their every report and thought stayed the same, since the functional state is identical. A mind that cannot notice its own experiences changing is not a coherent picture of a mind.
Criticism
- A substrate theorist can deny the premise that a silicon device could reproduce a neuron's input-output behaviour. This commits them to saying what a neuron does that physics cannot model, and none has done so.
- John Searle accepts the setup and bites the bullet: the subject's experience fades while their behaviour stays the same, and they are simply wrong about it. Chalmers' reply, which I share, is that this makes the subject's introspective reports false in a way that nothing in their cognition could register, which is a stranger claim than the one it is trying to avoid.
- My view: this is the argument that settles Substrate Independence for me, and I find it telling that it comes from the most careful defender of the hard problem. Chalmers and I disagree about whether physics explains experience; we agree that the material is not what matters.
Source
Chalmers, D. (1995). "Absent Qualia, Fading Qualia, Dancing Qualia", in T. Metzinger (ed.), Conscious Experience. — https://consc.net/papers/qualia.html