Be careful with thought experiments

lukeprog

Thagard (2012) contains a nicely compact passage on thought experiments:

Grisdale’s (2010) discussion of modern conceptions of water refutes a highly influential thought experiment that the meaning of water is largely a matter of reference to the world rather than mental representation. Putnam (1975) invited people to consider a planet, Twin Earth, that is a near duplicate of our own. The only difference is that on Twin Earth water is a more complicated substance XYZ rather than H2O. Water on Twin Earth is imagined to be indistinguishable from H2O, so people have the same mental representation of it. Nevertheless, according to Putnam, the meaning of the concept water on Twin Earth is different because it refers to XYZ rather than H2O. Putnam’s famous conclusion is that “meaning just ain’t in the head.”

The apparent conceivability of Twin Earth as identical to Earth except for the different constitution of water depends on ignorance of chemistry. As Grisdale (2010) documents, even a slight change in the chemical constitution of water produces dramatic changes in its effects. If normal hydrogen is replaced by different isotopes, deuterium or tritium, the water molecule markedly changes its chemical properties. Life would be impossible if H2O were replaced by heavy water, D2O or T2O; and compounds made of elements different from hydrogen and oxygen would be even more different in their properties. Hence Putnam’s thought experiment is scientifically incoherent: If water were not H2O, Twin Earth would not be at all like Earth. [See also Universal Fire. --Luke]

This incoherence should serve as a warning to philosophers who try to base theories on thought experiments, a practice I have criticized in relation to concepts of mind (Thagard, 2010a, ch. 2). Some philosophers have thought that the nonmaterial nature of consciousness is shown by their ability to imagine beings (zombies) who are physically just like people but who lack consciousness. It is entirely likely, however, that once the brain mechanisms that produce consciousness are better understood, it will become clear that zombies are as fanciful as Putnam’s XYZ. Just as imagining that water is XYZ is a sign only of ignorance of chemistry, imagining that consciousness is nonbiological may well turn out to reveal ignorance rather than some profound conceptual truth about the nature of mind. Of course, the hypothesis that consciousness is a brain process is not part of most people’s everyday concept of consciousness, but psychological concepts can progress just like ones in physics and chemistry. [See also the Zombies Sequence. --Luke]

Thagard (2012) contains a nicely compact passage on thought experiments:

Grisdale’s (2010) discussion of modern conceptions of water refutes a highly influential thought experiment that the meaning of water is largely a matter of reference to the world rather than mental representation. Putnam (1975) invited people to consider a planet, Twin Earth, that is a near duplicate of our own. The only difference is that on Twin Earth water is a more complicated substance XYZ rather than H2O. Water on Twin Earth is imagined to be indistinguishable from H2O, so people have the same mental representation of it. Nevertheless, according to Putnam, the meaning of the concept water on Twin Earth is different because it refers to XYZ rather than H2O. Putnam’s famous conclusion is that “meaning just ain’t in the head.”

The apparent conceivability of Twin Earth as identical to Earth except for the different constitution of water depends on ignorance of chemistry. As Grisdale (2010) documents, even a slight change in the chemical constitution of water produces dramatic changes in its effects. If normal hydrogen is replaced by different isotopes, deuterium or tritium, the water molecule markedly changes its chemical properties. Life would be impossible if H2O were replaced by heavy water, D2O or T2O; and compounds made of elements different from hydrogen and oxygen would be even more different in their properties. Hence Putnam’s thought experiment is scientifically incoherent: If water were not H2O, Twin Earth would not be at all like Earth. [See also Universal Fire. --Luke]

This incoherence should serve as a warning to philosophers who try to base theories on thought experiments, a practice I have criticized in relation to concepts of mind (Thagard, 2010a, ch. 2). Some philosophers have thought that the nonmaterial nature of consciousness is shown by their ability to imagine beings (zombies) who are physically just like people but who lack consciousness. It is entirely likely, however, that once the brain mechanisms that produce consciousness are better understood, it will become clear that zombies are as fanciful as Putnam’s XYZ. Just as imagining that water is XYZ is a sign only of ignorance of chemistry, imagining that consciousness is nonbiological may well turn out to reveal ignorance rather than some profound conceptual truth about the nature of mind. Of course, the hypothesis that consciousness is a brain process is not part of most people’s everyday concept of consciousness, but psychological concepts can progress just like ones in physics and chemistry. [See also the Zombies Sequence. --Luke]

Pointing out that biochemistry couldn't be the same if water was different sounds like deliberately missing the point of Putnam's experiment. Suppose a planet like Earth, but where most people are left-handed, have their heart in the right-hand side of their body, wear wedding rings on their right hand, most live in the hemisphere where shadows move counterclockwise, most screws are left-handed, conservative political parties traditionally sit in the left-hand side of assemblies, etc., etc., and they speak a language identical to English except that left means ‘right’, right means ‘left’, clockwise means ‘counterclockwise’, etc. (Throughout this post, I use upright type for actual English and italic type for the language of that planet.) BTW, they are made of the same kind of matter as us, so that in their language lepton does mean ‘lepton’, antilepton means ‘antilepton’, etc. Now, when someone on that planet (who's not a particle physicist) says left he's thinking the same thoughts as someone on this planet saying ‘left’, but it doesn't follow that left in their language means the same as ‘left’ in our language (because the statement in their language only right-handed leptons and left-handed antileptons participate in the weak interaction is true, but the statement in our language “only right-handed leptons and left-handed antileptons participate in the weak interaction” is false)... or does it?

Bar pbhyq nethr gung vs fbzrbar vf abg n cnegvpyr culfvpvfg gurve ynathntr qbrfa'g npghnyyl unir n jbeq sbe yrcgba rgp. Ohg vs jr tb gbb sne qbja guvf ebnq, gur vzcyvpngvba vf gung vs V gryy fbzrbar gb ghea yrsg, gura vs gurl qba'g xabj nobhg cnevgl ivbyngvba va jrnx vagrenpgvbaf jung V zrna ol ‘yrsg’ vf abg jung gurl zrna ol ‘yrsg’, juvpu frrzf hafngvfsnpgbel gb zr orpnhfr nsgre nyy gurl qb raq hc gheavat jurer V jnagrq gurz gb ghea.

Well, 'left' means 'right' and 'right' means 'left', right? That their macroscopic world is a parity-inverted copy of ours (and that their word for 'left' souds the same as our word for 'right') is an unfortunate confusing accident, but I don't see how it would justify translating 'left' as 'left'. The representation of 'left' in their brains is not the same as the representation of 'left' in our brains, as demonstrated by different reactions to same sensory inputs. If you show the twin-earther your left hand they would say "it's your right hand". In the H2O-XYZ counterfactual the mental representations could be the same, thus Putnam's experiment is different from yours.