Natural Kind Externalism and Twin Earth

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Natural Kind Externalism and Twin Earth

• Advanced Topics in Mind and Knowledge Lecture 2

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Recap of What is Mental Content?

• Mental content is
• Abstract
• Semantically evaluable (i.e. true or false)
• May or may not be mind-dependent.
• Privileged access to our own thoughts seems to
  suggest their content is mind-dependent.

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Twin Earth

• Putnam, however, uses a number of examples to
  argue against this.
• Although Putnam concerned with linguistic
  content, we can equally apply his arguments to
  mental content.
• Well concentrate on the most famous example, of
  Twin Earth.
• First, describe the thought experiment, then
  reconstruct Putnams argument.

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The Thought Experiment I

• Twin Earth is identical to Earth in every way
  except that on Twin Earth the liquid which is
  called water (and is clear, wet, quenches thirst
  etc) has a different chemical formula, XYZ.
• A visitor from Earth who found this out would say
  On Twin Earth water means XYZ and vice-versa.

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The Thought Experiment II

• But what about pre-1750?
• On both planets when people said water they
  meant the same thing as water post-1750.
• Imagine Oscar on Earth and Twin Oscar on Twin
  Earth are doppelgangers intrinsically identical
  pre-1750.
• When Oscar says water he means H2O, when Twin
  Oscar says water he means XYZ.
• Therefore, meanings aint (just) in the head!

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The Target

• Arguing against descriptivist theories of
  meaning, which hold that
• Meaning determines reference
• Meanings are determined by psychological states
• Putnam aims to show that these principles are
  inconsistent, and (2) is false.
• Intention was to replace descriptivism with a
  causal theory of reference, now widely accepted

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Putnams Argument

• We can reconstruct the argument as follows
• P1. Meaning determines reference.
• P2. Oscar and Twin Oscar are identical, and so
  share all their narrow states.
• P3. When O and TO say Water is wet, the two
  sentences have different truth conditions, since
  Os water is H2O and TOs is XYZ.
• C. The states which O and TO share do not
  determine the reference (P2, P3), and hence
  meaning (P1) of their words. Meanings are partly
  determined by external factors!

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• Thought experiment applies only to natural kinds,
  e.g. animals, minerals, vegetables.
• Nevertheless, expresses what is now considered to
  be a strong externalist intuition.
• Has far-ranging consequences, not only in
  epistemology and philosophy of mind (e.g.
  commensurability of scientific terms).

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Some Internalist Responses I

• Twin Earth is physically impossible, because
  nothing with radically different microstructure
  from H2O could have the same properties.
• Change the example to actual ones, e.g. jadeite
  and nephrite or aluminium and molybdenum. Closely
  similar macro properties, different micro ones.
• By water both mean H2O, or XYZ.
• But how could Oscar have meant XYZ when hes
  never encountered any? Also, choosing either way
  seems arbitrary.

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Some Internalist Responses II

• By water Oscar and Twin Oscar mean H2O and XYZ.
  So water isnt a natural kind concept it
  picks out two of them.
• Does this force us to give up P1? Perhaps not
  (Crane 1991).
• Does this response generalise?
• Perhaps water only became a natural kind
  concept after 1750. So we can correct uninformed
  uses, but only because we know about
  microstructure.
• Seems implausible that we cant have a natural
  kind concept without knowledge of microstructure.

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Some Internalist Responses III

• Water fails to refer to anything since there is
  no one natural kind which satisfies it. So all
  our beliefs about water are false on both Earths.
• But wed have to say the same about e.g. jadeite
  and nephrite. (Is this a serious problem?)

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Extending Twin Earth

• Putnams Twin Earth thought experiment is
  designed to show that linguistic meaning depends
  at least partly on the environment, in cases
  where we use natural kind concepts.
• Next week well be looking at Burges Arthritis
  argument which aims to extend this externalist
  conclusion to mental content, and to cases
  involving social concepts.