The vulgar riot and debauchery, which scarcely disgraced an Alcibiades or a Cæsar, have been exchanged for the higher ideals of a Bayard or a Sydney.—Pearson
With abstract nouns.
190. An or a before abstract nouns often changes them to half abstract: the idea of quality remains, but the word now denotes only one instance or example of things possessing the quality.
Become half abstract.
The simple perception of natural forms is a delight.—Emerson
If thou hadst a sorrow of thine own, the brook might tell thee of it.—Hawthorne
In the first sentence, instead of the general abstract notion of delight, which cannot be singular or plural, a delight means one thing delightful, and implies others having the same quality.
So a sorrow means one cause of sorrow, implying that there are other things that bring sorrow.
Become pure class nouns.
NOTE.—Some abstract nouns become common class nouns with the indefinite article, referring simply to persons; thus,—