Full Idea
We must distinguish the 'must' of necessity as applied to a proposition or state of affairs (de dicto) from the 'must' of essence, concerning the way in which an object has an attribute (de re).
Gist of Idea
We must distinguish the de dicto 'must' of propositions from the de re 'must' of essence
Source
Peter Simons (Parts [1987], 7.1)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.257
A Reaction
A helfpful distinction, but a possible confusion of necessity and essentiality (Simons knows this). Modern logicians seem to run them together, because they only care about identity. I don't, because I care about explanations.