Full Idea
If □ is to be sensitive to the quality of the truth of a proposition in its scope, then it must be sensitive as to whether an object is picked out by an essential property or by a contingent one.
Gist of Idea
□ must be sensitive as to whether it picks out an object by essential or by contingent properties
Source
M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 4.3)
Book Reference
Fitting,M/Mendelsohn,R: 'First-Order Modal Logic' [Synthese 1998], p.87
A Reaction
This incredibly simple idea strikes me as being powerful and important. ...However, creating illustrative examples leaves me in a state of confusion. You try it. They cite '9' and 'number of planets'. But is it just nominal essence? '9' must be 9.