Full Idea
A term is apt to be vague if it is to be learned by ostension, since its applicability must admit of being judged on the spot and so cannot hinge of fine distinctions laboriously drawn.
Clarification
'Ostension' is by picking out an instance
Gist of Idea
Terms learned by ostension tend to be vague, because that must be quick and unrefined
Source
Willard Quine (What Price Bivalence? [1981], p.32)
Book Reference
Quine,Willard: 'Theories and Things' [Harvard 1981], p.32
A Reaction
[Quine cites C. Wright for this] Presumably precision can steadily increased by repeated ostension. After the first 'dog' it's pretty vague; after hundreds of them we are pretty clear about it. Long observation of borderline 'clouds' could do the same.