Full Idea
There seems to be surface synonymy between 'is essentially' and de re occurrences of 'is necessarily', but intersubstitution often fails to preserve sense (as in 'Winston is essentially a cyclist' and 'Winston is necessarily a cyclist').
Gist of Idea
'Is essentially' has a different meaning from 'is necessarily', as they often cannot be substituted
Source
Ruth Barcan Marcus (Essential Attribution [1971], p.193)
Book Reference
-: 'Nous' [-], p.193
A Reaction
Clearly the two sentences have different meanings, with 'essentially' being a comment about the nature of Winston, and 'necessarily' probably being a comment about the circumstances in which he finds himself. Very nice. See also Idea 11186.
Related Idea
Idea 11186 'Essentially' won't replace 'necessarily' for vacuous properties like snub-nosed or self-identical [Marcus (Barcan)]