|Necessity, philosophy: different kinds of necessity are distinguished, differing in their strength. For example, physical, logical or metaphysical necessity. See also necessity de dicto, necessity de re.|
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|Schwarz I 27
Definition strong necessity/Chalmers: Thesis: In addition to substantial contingent truths, there are also substantial modal truths: For example, Kripke is essentially a human being, e.g. that pain is essentially identical to XY.
N.B.: knowledge of contingent facts is not sufficient to recognize these modal facts. How do we recognize them, perhaps we cannot do this (van Inwagen 1998) or only hypothetically through methodological considerations (Block/Stalnaker 1999).
Schwarz I 208
A posteriori/Necessity/Lewis/Schwarz: here the secondary truth conditions are generally fulfilled, but not the primary ones! The first circumstance makes the sentences necessary - secondary truths reflect the behavior in modal embeddings - the second makes them a posteriori. But not because primary conditions of truth would be determined by embedding in epistemic operators (as in (Chalmers, 2003)), but because, according to our language conventions, e.g. "The Morning Star is the Evening Star" may not always be expressed, but only when certain conditions are available about which we must first inform ourselves.
Schwarz I 209
E.g. if the astronomers announce tomorrow that the Morning Star is not the Evening Star, then they have real news, but they do not violate our language conventions. This has something to do with Lewis' description theory of the reference.
Chalmers I 63
Necessary Truth/Gareth Evans/Chalmers: (Evans 1979):
Definition "superficial necessity"/Evans: E..g "Water is H2O" when the modal operator is "actually fixed", i.e. related to the actual world (The world of the speaker). (Davies and Humberstone, 1980). It may turn out that the reference is different. (I.e., that it was different all the time).
Definition "deep necessity"/Evans: this is not influenced by a posteriori considerations.
These types of necessity and possibility refer to statements, not to worlds.
Truths conditions/Evans/Chalmers: Thus, two sets of truth conditions are associated with each statement (primary and secondary,> secondary intension/Chalmers).
Strong metaphysical necessity/Chalmers: would be one that assumes that it would be metaphysically impossible for a world to be identical with ours in regard to the physical facts, but not for all positive facts.
This is stronger than Kripke's metaphysical necessity, which we may call weak metaphysical necessity.
Conceivability/Chalmers: then worlds are conceivable that are not possible at all. Strong metaphysical necessity goes beyond the limitations we have described as "wrongly described worlds". Then "Zombie world" could correctly describe a world that we imagine, even with regard to a secondary intension. It is only the case that such a world would not be metaphysically possible.
1. Vs: there is no reason to believe that there is such a modality of metaphysical necessity. There are no analogies to this of how they are provided by examples such as water/H2O or Hesperus/Phosphorus, since they require only one possible world.
A posteriori Information: always affects only our own world! This can help to locate our world in the space of possible worlds.
2. Vs: If we allow this kind of metaphysical necessity, we open the door for further ad hoc modalities.
Zombie World: someone who believes that a zombie world is logically possible but metaphysically impossible, cannot answer the key question: Why could not God have created a Zombie world? If he had created it, it would still be metaphysically impossible. This is too arbitrary.
The Conscious Mind Oxford New York 1996
Constructing the World Oxford 2014
David Lewis Bielefeld 2005