Ambiguity is one way in which the meanings of words and phrases can be unclear, but there is another way, which is different from ambiguity, namely vagueness. This is best explained by the use of a stock example: 'bald'. Some men are definitely bald, and there is no debating the matter; for example, Patrick Stewart (Captain Picard of Star Trek). Other men are definitely not bald; for example, Bill Clinton. Then there are quite a few men who carefully comb a scanty amount of hair over their scalps, about whom we are not sure whether to say they are bald or not. There just is not any clear line between being bald and not being bald.
Another good example of a vague concept is the concept of a heap--two or three grains of sand is not a heap, but a thousand is. So how many grains of sand does it take to make a heap? There is no clear line. (See the paradox of the heap.)
When we look at a man with thinning hair or a small pile of sand, and we do not know whether to call the man 'bald', or the sand a 'heap', then we have found a borderline case. So we can make a general principle, which in fact might work as a definition of the word 'vague':
- To say a term, or a concept, is vague is to say it has, in addition to clear cases where it does and does not apply, borderline cases as well; in other words, cases where there is no clear fact of the matter whether the concept applies or not.
Consider those animals in Alaska that are the result of breeding Huskies and wolves: are they dogs? It is not clear: so we can say that they are borderline cases of dogs. This is not to say they are dogs, and it is not to say that they are not dogs. It is to say that our ordinary concept of doghood is not clear enough to let us rule conclusively in this case.
It turns out that vagueness is important philosophically. Suppose we want to come up with a definition of 'right' in a moral sense of this term. Generally, we can say that we want a definition to cover actions that are clearly right and exclude actions who are clearly wrong; but then what do we do with the borderline cases of actions that are neither clearly right nor clearly wrong? Surely there are such cases. What philosophers often say is that we should try to come up with a definition that is itself unclear on just those cases that are borderline cases. Others say that we have an interest in making our definitions more precise than ordinary language, or our ordinary concepts, would by themselves allow; they recommend that we advance so-called precising definitions. So some philosophers want their definitions to be unclear in precisely those areas in which the ordinary concept to be defined is unclear; while other philosophers want their definitions to be more precise than the ordinary concepts.