Full Idea
If we specify a theory with all of its terms, and then replace all of those terms with variables, we can then say that some n-tuples of entities can satisfy this formula. This Ramsey sentence then says the theory is realised, without specifying by what.
Gist of Idea
A Ramsey sentence just asserts that a theory can be realised, without saying by what
Source
David Lewis (How to Define Theoretical Terms [1970], II)
Book Reference
Lewis,David: 'Philosophical Papers Vol.1' [OUP 1983], p.81
A Reaction
[I have compressed Lewis, and cut out the symbolism]