Kripke's discovery of a posteriori necessity is often invoked as a great discovery in 20th Century Analytic Philosophy. I think it was an important discovery--just not what some seem to have thought it to be. Allow me to explain.
Recently, I have been arguing--along with people like Avner Baz and Mark Balaguer--that it is important not to confuse conceptual analysis with metaphysics. The traditional story of Kripke's discovery, if I have it right (and I may not), is that Kripke made a metaphysical discovery: that he discovered really interesting modal metaphysical facts (e.g. water is necessarily H2O) that we come to grasp through empirical discovery (i.e. water's molecular structure).
I want to suggest that this is not quite right.
Rather, I want to say, first, that Kripke only discovered (1) a couple of ordinary empirical facts which, when combined with (2) a simple a priori law (the law of identity), results in "a posteriori necessities." As such, I want to say that when looked at carefully, "a posteriori necessities" aren't very metaphysically interesting. They are nothing more than a couple of ordinary emprical facts combined with an ordinary a priori law of logic.
Second, I want to suggest that what Kripke found is akin to what Quine found with the analytic/synthetic distinction: namely, that just as there is no clean line between the analytic/synthetic, there is no clean line between the a priori and a posteriori.
Allow me to now explain both points.
Some commenters over at the Leiter thread on Searle's interview with Tim Crane have expressed some puzzlement over Searle's claim that he thinks philosophy of language should stick really close to the "psychological reality" of language usage and processing. What, some have asked, does he mean? Doesn't philosophy of language already do that (insofar as it is based on our judgments of what propositions, what names refer to, etc.)? Well, I don't know what Searle has in mind, but let me hazard a guess.
What, exactly, are "a posteriori necessary truths" supposed to be? Here is an example: "Hesperus is Phosphorous." Here is another: "Water is H2O." Why are these supposed to be necessary truths? Well, 'Hesperus' and 'Phosphorus' both refer to Venus, and it is necessary that Venus=Venus (since identity is necessary). Similarly, 'water' and 'H2O' both refer to the same molecular substance, H2O, and H2O=H2O. Okay, then, why are they supposed to be "a posteriori"? Answer: because we had to learn empirically that 'water' is that particular substance. We couldn't deduce it from the meaning of 'water.'
Okay then, so it looks as if Kripke's thought experiments have shown something very interesting. Whereas people had previously thought that necessary truths had to be analytic (viz. 2+2=4 is necessarily true analytically in virtue of its concepts), Kripke showed that some necessary truths are not analytic (again, we could not analytically deduce that water was H2O prior to studying the world). But now this looks metaphysically very interesting! We seem to have necessary truths about the world--modal metaphysical facts!
Or do we? Let's think about how a scientist might explain Kripke's discovery. That is to say, if we are looking at the empirical world--at brains, at water, how brains think, etc.--what is the best explanation of Kripke's discovery?
Here, I submit, is the answer.
First, Kripke discovered an ordinary empirical fact about language usage: namely, that we use the terms 'water' and 'H2O' as rigid designators (that is, to refer to the same self-identical stuff) across all possible worlds. Again, this is just an emprical fact. It is what we do. It is how, if Kripke is right, we modally use names.
Second, Kripke drew attention to an ordinary empirical fact about that stuff (the stuff we use 'water' and 'H2O' to refer to): namely, that its physical nature--its molecular structure--can be only known through empirical means (perception, microscopes, chemistry, etc.).
Third, and most obviously, Kripke invoked an ordinary logical/a priori modal fact: that the stuff 'water' and 'H2O' refer to across possible worlds--WATER!--is self-identical, and therefore necessarily self-identical (for Kripke, "water is H2O" is semantically equivalent to H2O=H2O, and A=A a priori entails necessarily A=A).
Putting all this together, we see the following. Kripke's discovery of "a posteriori necessities" was nothing other than a discovery of:
- An ordinary emprical fact about language usage (we use 'water' and 'H2O' to refer to the same thing across possible worlds).
- An ordinary empirical fact about what is invoved in knowing the referents of terms (we know what water is through perceiving it, and the structure of H2O through chemistry), combined with,
- An ordinary analytic/a priori logical fact (all things are necessarily self-identical).
If this right, then several things follow.
First, although Kripke discovered something philosophically interesting (two empirical facts plus the law of identity), he didn't discover anything metaphysically interesting. Again, the facts here are interesting! It is interesting that we use names as rigid designators. It is also interesting that we have to learn what water is by perception and chemistry. Finally, identity is an interesting logical notion. But none of these are metaphysical issues. They are simply two empirical points combined with a law of logic.
Second, this would seem to support Moti Mizrahi's view about essentialism in his paper, "Essentialism: Metaphysical or Psychological?". On the explanation of "a posteriori necessities" that I have just given, a posteriori necessities don't get their modal properties from the world outside of how we use language (plus, again, the law of self-identity). They get their modal properties from how we use language (and the law of self-identity)--which is to say, metaphysical "essences" aren't metaphysically in things (above and beyond self-identity), but rather constructed by us through rigid-designator-ish language usage.
Finally, on my analysis, "a posteriori necessities" are not fully a posteriori. Their modal properties are comprised by two interacting things: (1) an a posteriori fact (that we use names as rigid designators), plus (2) an a priori logical fact (all things are self-identical as a law of logic). What Kripke showed, in other words--like Quine's demolition of the analytic/synthetic distinction--is that there isn't any clean a priori/a posteriori distinction. "A posteriori necessities" aren't purely a posteriori. They have an a priori element (the law of identity). What does this mean? In some sense, it seems to me, it means that there is no clear division between what is "a priori" and what is "a posteriori." Many things that we want to call "a posteriori" truths --e.g. "Water is H2O" -- are partly a priori and a posteriori, thus in some sense "blurring the line" between which propositions are a priori and which are a posteriori. A better way to put it, perhaps, is that on the analysis I have just given, the proposition <Water is H2O> is necessarily true partly because of an a posteriori feature it has (we use names as rigid designators) and partly because of an a priori feature of it (the law of identity)--which is just to say that is not a "a posteriori necessity": it is an a priori/a posteriori necessity, having both elements at once.
Anyway, maybe all of this is old hat. I don't follow the philosophy of language literature very closely, but this was stuff that bugged me in graduate school, and it just occurred to me again, so I figured I'd share it.