"ex falso quodlibet, twin earth, philosophical method"

When we reason from premisses whose truth values are unknown, we often look to
their consequences in order to evaluate the truth of the premisses. (This can be
done formally, as in an attempt at proof by reductio, or otherwise.) However,
since false propositions imply anything (by ex falso) a mixture of true and
false propositions may be inferred accidentally. This mixture has to then be
culled to see if the premisses are reasonable and so on in a dialectic circle.

But what happens when the premisses are *known* to be false? Take one example of
a common premiss for various arguments about mental externalism, etc. in the
literature. I refer to the common "twin earth" type scenarios. Assume for the
sake of definiteness (if this is disputed, one can move the problem, I think,
but not overcome it) that twin earth is the hypothetical locale where the oceans
are not H2O (and instead "XYZ") and everything else the same.

Unfortunately, twin earth is nomologically impossible - hence the situation is
cannot occur, and any discussions of what things are like on this hypothetical
place include falsehoods. To see that twin earth is nomologically impossible,
recall some basic cell biology. On our earth, many single celled organisms feed
by forming vacuoles and producing enzymes and other substances into their
vacuoles when food is placed in it by various cellular processes. These cellular
processes bring in the food within an aqueous medium, suitably stripped of ions
and other chemical species by the cell membrane or wall, etc. Thus it is not
suprising that cellular fluid consists of mostly water - dihydrogen monoxide and
not "XYZ". Putnam's making up of a nonsensical chemical formula seems to obscure
the fact that dihydrogen monoxide has other "lawful roles" within earth's
environment, living and otherwise. Moving the XYZ to some definitive chemical
species shows the radical different life that would be found on twin earth if
(as we are doing) we assume the same laws apply. (I do not know what to say
about anything if we assume the laws of nature are different on twin earth -
such situations I will ignore because of the lack of constraints I see on our
theorizing.) For example, the life on twin earth can't eat the same way we do,
as I described above. It cannot form proteins by condensation reactions, as
these vitally require water as the medium to receive the "extra" water produced
in the reaction. If "XYZ" is the solvent of the oceans on twin earth, it thus
had better be a by product of the appropriate metabolic processes as well - and
then life looks *very* different. Here we enter the realm of extreme speculative
science fiction. And then we have the problem associated with the laws.

Thus the partisan of twin earth examples is caught in a dilemna. If he keeps
them simple, as Putnam did, they involve a knowingly false premiss. If they are
complicated, one has to spend one's time explaining how these creatures really
are or aren't like us, and how their psychology is somehow sufficiently like
ours - i.e. then one would have to defend a form of functionalism, no doubt -
but in any case get mired in other philosophical concerns far from what one was
originally concerned with. And, as the example of functionalism shows, we are
not too far away from philosophy of mind issues the twin earth examples was
supposed to illuminate, so we are in grave danger of begging the question.
Finally, even if we avoid this trap, then the twin earth material is almost
otiose - why not just tell the science fiction story from the get go and frame
it as such and thus defend it on its merits.

Moral: twin earth examples are dubious.

Objections answered: Someone might rejoin that I have spoken in actualist terms
about twin earth and I should speak subjunctively ("counterfactually"). The
answer to this is simple. Speak subjunctively if one wants: the problem is not
with the contrary-to-fact situation per se, it is that it is knowingly reasoning
from a false premiss. (Well, maybe Putnam and the others don't realize that they
have been, but now they do, assuming this article goes somewhere.) I suppose
someone will then say that this guts all thought experiments. See the third
objection.

Another objection (of the form "I told you so!") would be from Belnap and the
relevance logic school (or Priest and the paraconsistent guys). Yes, I see how
this is grist for your mill, Belnap/Priest. But we can talk about how radical a
response we need later.

Third objection answered: Someone says "But thought experiments have a long and
glorious history in philosophy and science. Why pick on twin earth?" Finding
that premisses in thought experiments are false in some contexts leads to
revising the thought experiment and its consequences as when Bohr showed that
Einstein (of all people) wasn't thinking relativistically at one point in a
thought experiment.

Fourth, continuing from the third: But even if Einstein put in the relativistic
correction in his debate with Bohr, there is still a false premiss, namely that
a situation like the one described obtained. This is correct too, and shows how
deep the puzzle is. My suggestion, tentative as it is, would appeal to the
difference between laws and boundary conditions. The laws in the revised
Einstein-Bohr thought experiments are (ex hypothesi) the ones in the real world,
whereas the situation may not be realized due to conditions. I realize that
getting out by appealing to what some regard as obscure (laws) is not helpful
too much, but at this point I am raising a worry, but not able to solve it.

Fifth: But, someone continues, it is fruitful to imagine counterfactually
varying the laws in order to check our knowledge of them. Agreed. And here I get
very puzzled indeed and do seem to be pulled in Belnap's direction, if not all
the way to Priest's. Then I read up on what that requires and I feel it can't be
done either. But that's another story for another time.

Sixth: I have supposedly run together logical possibility with nomological. I
reply: For one thing, logical possibility strictu sensu tells us nothing
whatsoever. It is *logically* possible that meanings are in the head, and not.
Why? Because logical possibility, as known for quite a while, is provability (or
satisfiability, whichever - it is really two notions, which complicates matters
inessentially here) within a given formal system. The sloppy pretheoretic notion
is just conceivability (or so it seems - nobody has ever told me what constrains
it), which for Putnam's uses is dangerous: so one can conceive of something? So
what? What is supposed to follow from that? People can conceive of
contradictions! It is here that a more rigorous notion of logical possibility is
needed; see earlier in this section. Further, I do not buy this idea that one
should care about mere logical possibility. I know of only one world, and some
approximation to some of its laws (= objective patterns of being and becoming).
But note also how fundamentally different the two various notions of possibility
are.

Sixth, bis: It seems that the appeal to nomological possibility versus logical
possibility is also a red herring. Why? Because one is *still* reasoning from a
false premiss. Is one supposed to simply imagine that the premisses are true?
That doesn't work, or how would one ever use reductio: one would be imagining
that the premisses are true, and true premisses entail true conclusions, so ...
If one is allowed to *pretend* they are true and yet draw false conclusions
because they *really aren't*, the problem arises again. If the conclusion is to
merely be that it is conceivable that meanings ain't in the head, or that the
notion is not self-contradictory or contradictory within some system or other,
ok, sure, but so what then? These are extremely weak conclusions, one which are
likely concludable without all the song and dance of the thought experiment.
Besides, Putnam doesn't put the conclusions in such terms anyway.

Seventh: I have been told sometimes to "repair" the thought experiment by
allowing for a substance identical in all respects save its composition to
water. And yet everything else is the same. What? How does that work? How can
that scenario be even coherently described? If XYZ is identical in all its
properties to water, it must react with sodium to form sodium hydroxide! And
then it must contain oxygen or there's a nuclear reaction going on. Or maybe
there's something exactly like oxygen propertywise but not oxygen in XYZ. And so
on. Eventually one bottoms out (or goes on an infinite regress if there are
infinitely many hierarchy stages to particles, strings, whatever). These
enitities are exactly like their regular-earth counterparts except *what* now?
Composition at the merely chemical level only delays the question. (Shorter, if
you are a fan, which I am not, of supervenience: the different properties of
various particles supervene on their composition.) Oh, but then my critics say,
well, maybe the laws are different. Ok, but then you've gotten as far into
science fiction as before. Why think one's intuitions about the psychology, etc.
are at all reliable? Moreover, why repair the thought experiment at all? I think
this concedes my point! If it did not matter that one was reasoning from a false
premiss, why would one one need to repair at all? 

Eighth: One critic asked me to consider the possibility (conceivability?) that
the premisses are in fact true. Ignoring the fact that this is rather extreme
(and seems to gut our entire ability to use science to understand how the world
is), I don't see how this helps, unless, somehow, one thinks that *every single
thought experiment premiss* is in fact true (or could be determined to be true,
if one has such a theory of truth).