A Posteriori Necessity

Kripke Holds the following to be examples of a posteriori necessary truths:

1. Ancestry

2. Origin

3. Constitution

4. Identity claims containing rigid designators

5. Theoretical Identifications 

Essentialism

• Essential property of x - x wouldn't have been x without it.
• F an essential property, n a name - Fn a necessary claim, but a posteriori because only discoverable a posteriori.

Scepticism about Essentialism

• De dicto/de re
• De dicto -- modal prefixes takes wide scope; truth depends on descriptions, not the world. Known a priori. Eg. 'Necessariyl the inventor of the zip invented the zip.'
• De re -- modal prefixes take narrow scope; truth depends on nature of the thing, eg. 'the inventor of the zip necessarily invented the zip.'

• Quine's criticism of essentialism:
• De re modality is unintelligible; necessity and possibility purely a matter of description
• Example: number 9 necessarily greater than 7.
• True when described as '7+2', not so when 'the number of planets'.
• Similar case applies with mathematicians who are also cyclists 
• Response: counterintuitive; we can just say of 'the object' referred to by all the descriptions - can that fail to be greater than 7?

A Posteriori Necessity

1. Essentiality of Ancestry 

The Queen, Elisabeth II, couldn't have had different parents.

Could the Trumans have been her parents? No - at most, their child would have resembled Elisabeth II; she may even have gone on to become the Queen.

Objections:

1. Line of Questioning

Intuitons about possibility are sensitive to the line of questioning. There are two ways of phrasing the question:

a) 'Could this very woman have different parents?'

b) 'Could Elisabeth II have had different parents?'

a introduces criteria.

2. Contrary intuitons

I might say 'if I'd had different parents, things might've turned out differently.'

Two conflicting intuitions; Kripke offers no grounds for favouring one.

Argument two for Ancestry: 

• Alternate possibilities as actual course of history with eventual divergence.
• eg. Nixon might never have been a politician because, at some point in his life, this might have been true.
• Couldn't say of Nixon that his parents might have been different.
• At no point in his life could A and B have not been his parents.
• Objection - proves too much, eg. implies location of conception must be necessary.

2. Essentiality of Material Origin

• Could this table have been made from a different piece of wood, or different material altogether?

Ahmed breaks down Kripke's argument

1. Table A is made from block B.

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

3. Every possible world where block C…