Classic Poem

My First is singular at best:
 More plural is my Second:
My Third is far the pluralest—
So plural-plural, I protest
 It scarcely can be reckoned!

My First is followed by a bird:
 My Second by believers
In magic art: my simple Third
Follows, too often, hopes absurd
 And plausible deceivers.

My First to get at wisdom tries—
 A failure melancholy!
My Second men revered as wise:
My Third from heights of wisdom flies
 To depths of frantic folly.

My First is ageing day by day:
 My Second's age is ended:
My Third enjoys an age, they say,
That never seems to fade away,
 Through centuries extended.

My Whole? I need a poet's pen
 To paint her myriad phases:
The monarch, and the slave, of men—
A mountain-summit, and a den
 Of dark and deadly mazes—

A flashing light—a fleeting shade—
 Beginning, end, and middle
Of all that human art hath made
Or wit devised! Go, seek her aid,
 If you would read my riddle!

Answer: Imagination (I-Magi-nation).