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ArgumentThe multiple-realizability argument
What makes a mental state is the role it plays, not the stuff that realizes it: and the same role can be played by very different materials.
Supports
Putnam · "Psychological Predicates" (later retitled "The Nature of Mental States") · 1967
- We never ascribe a mental state by inspecting a material: we ascribe it to the role it plays in what a being does. It avoids what hurt it, hesitates, learns, corrects itself. That is how we recognize a dog's pain and a child's fear.
- Yet the same role is played by very different materials and architectures, which is called : the octopus, whose nervous system is nothing like ours, hesitates and learns; and in humans themselves, after a lesion, a function relocates elsewhere in the brain without the corresponding experience disappearing.
- What varies while the mental state stays the same is not what constitutes it: if the substrate can vary while pain remains pain, it is not the substrate that makes the pain.
- What remains is what did not vary: the organization, that is, a network of relations between what comes in, what is transformed within, and what goes out. Nothing in that organization mentions carbon, neurons, or the living.
Therefore So demanding a particular material is not a philosophical requirement but a substrate prejudice: whatever realizes the organization thinks, silicon included.
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Deepenings
The octopus and plasticity
- The octopus has neurons, but its nervous system is spread through its arms, with nothing resembling our architecture. It nonetheless solves problems, recognizes people, learns by trial: a lineage separated from ours for six hundred million years realizes the same functions otherwise.
- And without leaving our own species: in a child whose hemisphere is removed, speech and memory re-establish themselves in the other. It was therefore not that particular tissue that carried those states, since another carries them.
- Demanding the material of the human brain would mean denying pain to everything that is not us: biology has already ruled against that demand.
Therefore Multiple realizability is not a philosopher's hypothesis: it is a fact of nature, which the machine merely extends.
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We never have any other proof
- Of another's mind I never have direct access: I perceive only bodies that speak and act. On that basis alone I grant everyone an inner life, without its striking me as risky.
- If a machine holds its place in those same exchanges, answering, explaining itself, correcting itself, then demanding an extra proof applies to it a criterion applied to no one else.
- In the , does not claim to prove that it thinks: he points out that we have no other criterion, and that failing to produce one, the refusal is merely a privilege we grant ourselves.
Therefore Denying thought to the machine requires a criterion we apply to none of our fellows.
How far does this reasoning convince you?
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Put to the test
Objection