1.2.3 One vs. a(n)

Rule: One used in place of a(n) emphasizes the fact that one of a definite number of items is being singled out for attention.

Example of error:

(1) Constant advertisement is a reason why so many people go-to see science fiction films nowadays.

Quirk et a!. (1972:@4.i26) describe the function of one as "a stressed variant of the indefinite article a(n)...in contrast with the dual two and both and the plural numerals three, four, etc; several; and indefinite some." But definiteness, as well as stress or emphasis, seems to be involved here. One seems to mean 'one of a definite number' as opposed to a(n) 'one of an indeterminate number'. (1) is a case in point, since it raises the question of why a is not welcome here when it is perfectly acceptable in a sentence like

Constant advertisement is a necessity these days.

The difference seems to lie in the semantics of the nouns: one can more easily allow for an indefinite number of necessities than of reasons. Reason, understood as the true cause or causes of something, predisposes us to think of one, two, three or some other definite number of causes or reasons. Necessity, on the other hand, does not contain this implicit appeal to objective truth, and thus to determinacy, so we are more comfortable referring to an open-ended or indefinite number of possibilities. This is not to say, of course, that a word like reason can only be used in a definite sense. Consider, for example, the following:

Can you give me a reason for your behavior?
There must be a reason for it.

The explanation here may be that reason in the first sentence has a slightly different meaning than in (l)--a more subjective meaning closer to 'explanation'. A may be acceptable in the second sentence because the numerical idea (one among a number) has given way to a purely existential one; i.e. the question of there either being or not being a reason at all obscures the possibility of there being more than one.