Full Idea
In some of our ideas there are certain relations, habitudes, and connexions, so visibly included in the nature of the ideas themselves, that we cannot conceive them separable from them, by any power whatsoever.
Gist of Idea
Some of our ideas contain relations which we cannot conceive to be absent
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 4.03.29)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.559
A Reaction
This is the conceptual version of a priori necessity. The question then becomes whether this necessity can be traced back to reality, or merely to conventions which created the ideas in the first place. Analytic philosophy likes this idea.