Kripke on a Posteriori Necessity

1. Ancestry

First argument - intuition: 

• Ancestry essential – eg. the Queen could not have had different parents. She could not have been born to the Trumans.

• So - the Queen's parents were necessarily the parents she actually had.

Problems:

1. Intuitions about what could/could not have been the case sensitive to the line of questioning that elicits them.

a) If I had asked 'would it have been that very woman'

b) If I had asked 'could Elisabeth II have had different parents'

'a' seems to presuppose criteria for the kind of thing that can count as 'the same woman as Elisabeth II'.

2. Intuitions to the contrary exist. I might have certain regrets, and say, 'if I had had different parents, I would have had more opportunities.' This intuition presupposes the possibility of different parents.

So we have two different conflicting modal intuitions, and no good reason for favouring one over the other. Kripke's preference seems to rest on a prior belief about origin (i.e. that it is essential); but the ground for this is the intuition it's being used to defend.

Second argument - divergence of history:

The notion of alternate possibilities can be understood as the actual course of history, which eventually diverges. For example, Nixon might never have been a politician because there was a time in his life when this might have been true.

Truly said of Nixon that he might have lacked P iff at some time the past it was true that Nixon might in the future fail to have P.

But - in no time in the future would Nixon's parents have not been A and B.

Problem:

Proves too much. Implies also that location of conception must be essential to one's identity.

2. Material Origin

Kripke argues that this table could not have been made from anything other than the block of wood it actually was made of. So, it couldn't have been made from some other piece of wood, or ice, etc.

Kripke's Argument Broken Down:

1. Table B is made from block A

2. If possible to make tables B and D from blocks A and C, possible to make both.

3. In every possible world where D is made from C, D does not equal B.

4. In some possible world w, B is made from block A, and D from some block C.

5. In w, B and D are distinct. (from 3, 4)

6. Any world where B exists, and D is made from C, D and B are distinct.

7. Any world where B exists and anything is made from C – that thing is distinct from B.

8. In every possible world,B is not made from C.

9. If table B is made from block A, then it can't have been made from block C.

Problem:

Through an analogous argument, Ahmed proves this to be invalid: it also implies that two people could not be married to the same person (i.e. if B is married to A, and D is married to…