Full Idea
A source of confusion is when any complex idea is made up of too small a number of simple ideas, and such only as are common to other things, whereby the differences that make it deserve a different name are left out.
Gist of Idea
Ideas are uncertain when they are unnamed, because too close to other ideas
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.29.07)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.365
A Reaction
In other words, a word covers a variety of entities, and so it cannot possibly pinpoint any of them exactly. Cats all differ, but so do small and large circles.