The Reductio of Strong Externalism
McKinsey's Maxim [1]:
For any propositions E and P the following holds. If E cannot be known a priori and theory X is committed to it being an a priori truth that if one is thinking that P, then E, then either one cannot know a priori that one is thinking that P or theory X is false.[2]
For example, let E and P be the following and lets call the theory, which is committed to it being an a priori truth that if one is thinking that P, then E, strong externalism (SE).
(E) I am not a Dry Earthian.
(P) I am thinking that water is a liquid.
(Dry Earth is a world qualitatively identical to Earth but on Dry Earth all the watery stuff is an illusion.)
Principle of Privileged Access (PA): A normal subject can know what they are thinking by introspection alone.
If the fact that one is not a Dry Earthian cannot be known a priori and strong externalism is committed to it being an a priori truth that if one is thinking that water is a liquid, then they are not a Dry Earthian, then either one cannot know a priori that they are thinking that water is a liquid or it is not an a priori truth that if one is thinking that water is a liquid, then they are not a Dry Earthian. Since giving up (P) would violate (PA) it is clear that strong externalism is incompatible with privileged access.
Using the logic of McKinsey's Maxim we can get the following set up for a reductio ad absurdum of strong externalist compatiblism (SEC) (who is committed to the conjunction of (SE) and (PA).
1s. (SEC) --> (SE) & (PA)
2s. (SE) --> [(P) --> (E)]
3s. (PA) --> (P)
Cs. (E)
The strong externalist compatibilist is committed to both (SE) and (PA), i.e. they are committed to it being an a priori truth that if one is thinking that (P), then (E) and that it is knowable a priori that one is thinking that (P). Thus, it follows that according to (SEC) one can know a priori that (E). Assuming (SEC) the reductio can be written as follows.
(1) If I am thinking that water is a liquid, then I am not a Dry Earthian.
(2) I am thinking that water is a liquid.
(3) Therefore, I am not a Dry Earthian.
The Weak Externalist
"Any externalist thesis that implies that it is a priori true that if one is thinking that P, then E, where E cannot be known a priori, is indeed incompatible with privileged access. Our position is just that no such externalist thesis has as a consequence that it is a priori knowable that if one is thinking that P, then E, where E is not itself knowable a priori."[3]
"Although one can know a priori that one is thinking a thought involving, say, the concept water, one cannot know a priori that this concept is a natural kind concept. It is epistemically possible that water might turn out to be a disjunction of two different substances, as has turned out to be the case with jade. Our reasons for believing that this is not in fact the case are empirical."[4]
When faced with the reductio the externalist should opt for a weaker claim by making the consequent of 2s conditionalized on (N).
(N) Water is a natural kind.
Now the argument looks like this with a weakened conclusion.
1w. (WEC) --> (WE) & (PA)
2w. (WE) --> [(N) --> {(P) --> (E)}]
3w. (PA) --> (P)
Cw. (N) --> (E)
The weak externalist compatibilist is committed to both (WE) and (PA), i.e. they are committed to it being an a priori truth that if water is a natural kind, then {if one is thinking that (P), then (E)} and that it is knowable a priori that one is thinking that (P). Thus, it follows that according to (WEC) one can only know a priori that if (N), then (E). The (WEC) argument can be written as follows.
(1*) If water is a natural kind, then {if I am thinking that water is a liquid, then I am not a Dry
Earthian}.
(2*) I am thinking that water is a liquid.
(3*) Therefore, if water is a natural kind, then I am not a Dry Earthian.
The Dry Earthians are entitled to the same reasoning as we are and all our other intrinsic twin inhabitants of possible worlds. We have all the same intuitions as they do about 'water', we all say if water turns out to be XYZ, then there is XYZ and all water is XYZ, etc. We all say that if it turns out that we are Dry Earthians, then 'water' is a functional kind, i.e. the watery stuff. These intuitions we share about the meaning of 'water' superviene on our identical intrinsic properties?Is this all the internalist wanted? No, the internalist wants the content of 'water' to superviene only intrinsic properties. Perhaps some content does so supervieve but thats not the only content. So that speaks in favor of externalism. On the other hand, the fact that the content of 'water' supervienes on the intrinsic properties of a subject (plus a world) seems to at least be in the spirit of internalism, e.g. it is our semantic intentions that determine the content of 'water' given a world as argument. The external conditions play an important role in individuating content but it is kind of secondary to our semantic intentions. At this point I start to think that externalism is either an absurd thesis like SE above or a very trivial thesis.
[1] McKinsey (1991).
[2] (MM): [~KE & (X --> (KP --> KE)] --> [~KP or ~X], where KE = know a priori that E, X = theory X and KP = know a priori that one is thinking P.
[3] Tye & McLaughlin (1998).
[4] Falvey (2000), p. 139.
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