John Bunyan's accident of fiction

"When at the first I took my pen in hand
Thus for to write, I did not understand
That I at all should make a little book
In such a mode; nay, I had undertook
To make another; which, when almost done,
Before I was aware, I this begun.

"And thus it was: I, writing on the way
And race of saints, in this our gospel day,
Fell suddenly into an allegory"

-- John Bunyan, "The Author's Apology for his Book," Pilgrim's Progress