Externalism about mental content is the thesis that what one’s mental content (at least where atomic natural kind concepts are invoked) is individuated in part on the nature of that individual’s physical and social environment (what is out there and how words are used) – things external to the cognizer. In other words, we could fix everything internal to the cognizer, yet by changing things in her physical and social environment the content of at least some of her thoughts could be altered. Twin-Earth thought experiments are used to motivate externalism about mental content.
This line of thought seems to be problematic for the traditional doctrine of privileged self-knowledge. The idea here is that without the aid of empirical investigation, if one is thinking about p, then one can know that one is thinking about p. The claim is that I can know a priori that I am having a thought (when I am) and that my thought has the particular content that it does, for instance: that water is wet.
Here is an argument that externalism and privileged access are incompatible:
1. It is a priori knowable that that if one is thinking that water is wet, then one has the concept of water.
2. It is a priori knowable that the concept of water is an atomic natural kind concept.
3. It is a priori knowable that if a concept is an atomic natural kind concept then it is metaphysically impossible to possess it without having interacted with instances of that concept.
4. Therefore, it is a priori knowable that if one is thinking that water is wet, then one has causally interacted with instances of water.
5. It is not a priori knowable that one has causally interacted with instances of water.
6. Therefore, it is not a priori knowable by one that one is thinking that water is wet.
Premise (1) seems true since one must possess the concepts ingredient to the propositions that one thinks. I would fail to have the thought that water is wet if I failed to possess one or more of the concepts ingredient to the proposition water is wet.
Premise (3) follows from externalism about mental content. Which atomic natural kind concept one possesses is determined at least in part by external (physical and/or social) environmental factors.
Premise (4) is a sub-conclusion that follows from premises (1) – (3).
Premise (5) is clearly true. One cannot know a priori that water exists or that one has causally interacted with it. Empirical investigation is needed to know either of these claims.
The conclusion, line (6), is incompatible with the privileged access thesis. It states that what one is thinking is not a priori knowable to them in this case.
The compatibility of externalism and privileged access thus rests on premise (2). The claim is that it is a conceptual truth that water is an atomic natural kind concept. An atomic concept is one that lacks conceptual constituents. While it can often be known a priori that a concept is not atomic, it is by no means clear that one can know a priori when a concept is atomic. As such, this gives us some reason to doubt the second premise. In addition, it is much more difficult to see how one is supposed to know a priori that the concept water is a natural kind concept. While water is a natural kind, it does not follow that this fact is knowable a priori, and it seems quite likely that such proposition is not knowable a priori. After all, it is epistemically possible that water not be a natural kind, but rather not be a kind, like air or jade; or even fail to have an extension at all, like phlogiston. In other words, one must know that water is ‘Twin-Earth eligible’, and this is something that cannot be known a priori.
The incompatibilist can respond, as Boghossian does, by claiming that so long as a term aims to be denote a natural kind, something that is knowable a priori, then it must have a non-empty extension. However, it is not clear why externalists are committed to the claim that concepts that aim to denote a natural kind are only possessed by individuals who have causally interacted with the extension of that concept. In other words, why take the externalist’s thesis about natural kind concepts to also apply to concepts that aim to denote natural kinds? Since concepts that aim to name a natural kind can fail to name a natural kind, it is hard to see how this response helps the incompatibilist since it broadens the externalist’s commitments.
Since the externalist’s claims are particular to atomic natural kind concepts, then in order for the argument to go through from the fact that I have a thought involving a concept to the existence of an extension of that concept, one must know (a priori) that the relevant concept is of the relevant kind (an atomic natural kind concept). We have see reason to doubt that this can be known a priori, and if this cannot be known a priori, then the argument for the incompatibility of externalism and privileged access as typically construed fails.