Similar behaviour supports an analogy with my own mind.
Belief in other minds is reasonable, but depends on extending one private case to everyone else.
Philosophy of mind
How can I know that other minds exist?
The problem in miniature
The perfect companion
An artificial companion remembers your conversations, laughs at the right moments, becomes quiet when hurt and describes vivid dreams. Its words and expressions are indistinguishable from those of a conscious friend. Its designers insist that it only processes inputs and generates outputs; the companion insists that it feels fear, pleasure and loneliness. Opening the case reveals mechanisms, but no visible experience. You cannot inspect another human's consciousness either. If behaviour is not enough, it is unclear what further evidence could ever settle the question.
Why this is difficult
The pressure comes from more than one direction.
Mental states are shown through meaningful behaviour.
A mind is not a hidden object behind action, though perfect imitation becomes difficult to classify.
Conscious experience is private and irreducible.
No amount of public evidence can deliver certainty about what another being feels.
Philosophers in the conversation
Different routes through the same problem.
John Stuart Mill
Mill develops an argument from analogy: other bodies act as his body does when he experiences thoughts and feelings, so similar behaviour supports belief in similar inner causes. The argument makes other minds probable, but begins from a single directly known case — one's own.
Ludwig Wittgenstein
Wittgenstein challenges the picture of mental language as labels attached to wholly private objects. Words for pain and emotion gain their use within shared human practices. Another person's pain is not inferred in every encounter as though it were an invisible object hidden behind their behaviour.
Gilbert Ryle
Ryle rejects the mind as a private ghost operating a public machine. To describe intelligence, belief or character is often to describe dispositions expressed across behaviour and circumstances. His approach narrows the apparent gulf between inner mind and outward life, but faces the possibility of convincing imitation.
Thomas Nagel
Nagel emphasises the subjective character of consciousness: there is something it is like for a conscious creature to exist. Objective knowledge of behaviour and physiology may grow without capturing that point of view, preserving a gap between evidence about a being and access to its experience.
Key ideas
Language for thinking more precisely.
- Other minds
- The problem of justifying belief that beings other than oneself have conscious experiences.
- Analogy
- Reasoning from the link between one's own behaviour and experience to similar cases.
- Behaviourism
- An approach that explains mental concepts through behaviour, dispositions or public criteria.
- Qualia
- The felt, first-person qualities of experiences such as pain or colour.
- Solipsism
- The position that only one's own mind is known, or perhaps exists.
- Philosophical zombie
- A conceivable physical and behavioural duplicate said to lack conscious experience.
Reconsider your answer
The opening question has changed.
Demanding proof of consciousness may set a standard that no human other than yourself could meet. Accepting behaviour as decisive may grant minds to systems that imitate us without feeling anything.
If you cannot prove that other minds exist, what makes treating others as conscious rational rather than merely polite?