Full Idea
Some philosophers make a metaphysical shift, by inventing objects (individual concepts, forms, substances) called 'essences', which have only essential properties, and then worry when they can't locate them by rummaging around in possible worlds.
Gist of Idea
If essences are objects with only essential properties, they are elusive in possible worlds
Source
Ruth Barcan Marcus (Essential Attribution [1971], p.192)
Book Reference
-: 'Nous' [-], p.192