There is an etymological convertibility, and a syntactic convertibility; and although, in some cases, the
line of demarcation is not easily drawn between them, the distinction is intelligible and convenient.
[§ 423]. Etymological convertibility.—The words then and than, now adverbs or conjunctions, were once cases: in other words, they have been converted from one part of speech to another. Or, they may even be said to be cases, at the present moment; although only in an historical point of view. For the practice of language, they are not only adverbs or conjunctions, but they are adverbs or conjunctions exclusively.
[§ 424]. Syntactic convertibility.—The combination to err, is at this moment an infinitive verb. Nevertheless it can be used as the equivalent to the substantive error.
To err is human = error is human. Now this is an instance of syntactic conversion. Of the two meanings, there is no doubt as to which is the primary one; which primary meaning is part and parcel of the language at this moment.
The infinitive, when used as a substantive, can be used in a singular form only.
To err = error; but we have no such form as to errs = errors. Nor is it wanted. The infinitive, in a substantival sense, always conveys a general statement, so that even when singular, it has a plural power; just as man is mortal = men are mortal.
[§ 425]. 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.