Ambiguity is important to logic and to careful thinking more generally because if a statement is ambiguous, it affects our ability to evaluate its truth-value. More specifically, if a sentence is ambiguous, then there are at least two distinct meanings that can be attributed to it, and these two meanings represent different statements. Therefore, until we rid sentences of ambiguity, we are not in a position to evaluate their truth-value. You might think of an ambiguous sentence as comparable to a picture or sign that conveys more than a single meaning.
Furthermore, when ambiguous sentences occur in the context of arguments (either as premises or conclusion), we cannot evaluate their contribution to the argument. Therefore, ridding sentences of ambiguity is a preliminary task that must be completed before you can successfully evaluate an argument.
Golden Rules of Disambiguation
1. You cannot disambiguate a sentence if you write fewer than two sentences, although you may locate more than two possible meanings attributable to the original sentence. So, write at least two unambiguous sentences.
2. Add no new meaning-bearing elements: this is a matter of being charitable to the speaker/writer, even if it means preserving an element or elements of vagueness, which is a separate issue. Indeed, a part of the rationale for learning how to disambiguate is to ensure that you no longer confuse ambiguity with vagueness.
3. Check to see that you have not re-introduced the original ambiguity by means of a change in syntax or even, say, synonyms.
4. Make sure that your resulting sentences contains no new ambiguities.
Types of Ambiguity
Syntactic ambiguity occurs whenever a sentence can be understood as having two or more distinct meanings as a result of the order of the words within the sentence. Examples:
He put the ketchup on himself.
He watched her paint with enthusiasm.
Semantic ambiguity occurs whenever one or more words within a sentence can be understood as having two or more distinct meanings. Examples:
She knew a little Greek.
He stood on the bank.
Grouping ambiguity occurs whenever the meaning of a sentence is affected by whether we understand a word or phrase to refer to individual members of a group, or a group as a whole. Examples:
Nurses make more money than doctors.
Hippos eat more than dogs eat.
Closely related to this type of ambiguity are the fallacies of composition and division.
The fallacy of composition occurs when somebody asserts that because the parts of x have a certain quality, property, or disposition, then so too must the whole of x. Example: "The storage units are small, so it must be a small storage unit company."
The fallacy of division occurs when somebody asserts that because the whole of x has a certain quality, property, or disposition, then so too must the parts of x. "Salt is edible, so I guess sodium and chlorine are both edible, too."
Verbal ambiguity occurs whenever a sentence admits of more than one meaning, but these multiple meanings are eliminated once the sentence is written down. For example, if I were to say, “He’s got quite a tail,” you would not necessarily know if I meant:
He’s got quite a tail, or
He’s got quite a tale.
Since I have written these out, however, you know immediately to which I am referring and thus the written form immediately eliminates the ambiguity.