[§ 486]. The adjective used as a substantive.—Of these, we have examples in expressions like the blacks of Africa—the bitters and sweets of life—all fours were put to the ground. These are true instances of conversion, and are proved to be so by the fact of their taking a plural form.
Let the blind lead the blind is not an instance of conversion. The word blind in both instances remains an adjective, and is shown to remain so by its being uninflected.
[§ 487]. Uninflected parts of speech, used as substantive.—When King Richard III. says, none of your ifs, he uses the word if as a substantive=expressions of doubt.
So in the expression one long now, the word now=present time.
[§ 488]. The convertibility of words in English is very great; and it is so because the structure of the language favours it. As few words have any peculiar signs expressive of their being particular parts of speech, interchange is easy, and conversion follows the logical association of ideas unimpeded.
The convertibility of words is in the inverse ratio to the amount of their inflection.