Full Idea
Many of our best words in philosophy do not admit of definition, the notion of metaphysical 'necessity' being one pertinent example.
Gist of Idea
The excellent notion of metaphysical 'necessity' cannot be defined
Source
Gideon Rosen (Metaphysical Dependence [2010], 03)
Book Reference
'Modality', ed/tr. Hale,B/Hoffman,A [OUP 2010], p.113
A Reaction
Rosen is busy defending words in metaphysics which cannot be pinned down with logical rigour. We are allowed to write □ for 'necessary', and it is accepted by logicians as being stable in a language.