Full Idea
Boundaryless concepts tend to come in systems of contraries: opposed pairs like child/adult, hot/cold, weak/strong, true/false, and complex systems of colour terms. ..Only a contrast with 'adult' will show what 'child' excludes.
Gist of Idea
Boundaryless concepts tend to come in pairs, such as child/adult, hot/cold
Source
Mark Sainsbury (Concepts without Boundaries [1990], §5)
Book Reference
'Vagueness: a Reader', ed/tr. Keefe,R /Smith,P [MIT 1999], p.258
A Reaction
This might be expected. It all comes down to the sorites problem, of when one thing turns into something else. If it won't merge into another category, then presumably the isolated concept stays applicable (until reality terminates it? End of sheep..).