The question is whether the article is a separate part of speech (i.e. a word) or a word-morpheme. If we treat the article as a word, we shall have to admit that English has only two articles - the and a/an. But if we treat the article as a word-morpheme, we shall have three articles - the, a/an, ø.
The articles, according to some linguists, do not form a grammatical category. The articles, they argue, do not belong to the same lexeme, and they do not have meaning common to them: a/an has the meaning of oneness, not found in the, which has a demonstrative meaning.
If we treat the article as a morpheme, then we shall have to set up a grammatical category in the noun, the category of determination.