[hist-analytic] Kripke on Franklin and bifocals

Roger Bishop Jones rbj at rbjones.com
Tue Oct 27 17:10:59 EDT 2009

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I now offer a diagnosis of the fallacy in Kripke's argument for the claim that 
some propositions are necessary but a posteriori.
The argument I criticise is the one presented in Aune ETK chapter 2, though in 
this case Bruce presents it as being Kripke's original argument rather than 
his own variant.

Accepting for the sake of the argument the supposition that the designators 
"Franklin" and "the man who invented bifocals" are both rigid designators in 
our language and designate the same individual, I concur with Kripke in 
judging that the claim "Franklin is the man who invented bifocals" is 
necessary, but I argue contra Kripke (and Aune) that it is also a priori.

Kripke's argues fallaciously that since the identity of the inventor of 
bifocals is contingent, the claim must be a posteriori.
His fallacy lies in not distinguishing between information required to 
establish what proposition is expressed by the claim and information needed to 
establish the truth of the proposition.

The meanings of natural languages are contingent.
If the use of a contingent proposition about the meaning of a sentence 
debarred knowledge of its truth from being a priori then no sentence in a 
natural language could be known a priori.

In making a judgement about whether a proposition expressed by some sentence 
is a priori or a posteriori, it is first necessary to establish what the 
proposition is, and then to ask how that proposition could be justified.  If 
the meaning of a sentence is contingent then the connection between the 
sentence and the proposition it expresses will be contingent, and will be a 
posteriori.  However this does not debar the proposition itself from being a 

The proposition expressed by the claim:

	"Franklin was the inventor of bifocals"

(subject to the assumption that the two designators are rigid)
is simply an instance of the reflexivity of identity (this fact is used in the 
argument to establish its necessity), and hence can be known a priori.  The 
contingent fact that Franklin did invent bifocals is relevant only to 
establishing the meaning of the sentence, i.e. the proposition expressed, and 
is not material to its truth.

Roger Jones
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