Full Idea
A proposition essentially contains a given constituent if its replacement by some other constituent induces a shift in truth value. Thus Socrates is essential to the proposition that Socrates is a philosopher, but not to Socrates is self-identical.
Gist of Idea
A proposition ingredient is 'essential' if changing it would change the truth-value
Source
Kit Fine (The Question of Realism [2001], 6)
Book Reference
-: 'Philosophers' Imprint' [-], p.18
A Reaction
In this view the replacement of 'is' by 'isn't' would make 'is' (or affirmation) part of the essence of most propositions. This is about linguistic essence, rather than real essence. It has the potential to be trivial. Replace 'slightly' by 'fairly'?