Full Idea
To define theoretical terms might be to show how to do without them, but it is better to say that it shows there is no good reason to want to do without them.
Gist of Idea
Defining terms either enables elimination, or shows that they don't require elimination
Source
David Lewis (How to Define Theoretical Terms [1970], Intro)
Book Reference
Lewis,David: 'Philosophical Papers Vol.1' [OUP 1983], p.78