Who was the father of our country, Sidney?

In the NYT today, William Safire writes:

Quote:

Let me recall this gag, a literary chestnut but apropos:
What is the question to which �Washington Irving� is the answer?
The question: �Who was the father of our country, Sidney?

I don't get it. Help, please!

Sydney and Irving are both ethnic names its easy to pair them in a joke..

who was the father of our country, sidney?(asks Irving)
Washington, Irving (replies Sidney)

it seems that Safire, for one, has been promulgating this lame joke for some time.

Wasn't Yossarian confused about Washington Irving or Irving Washington in Catch 22?

There was a Washington Irving/Irving Washington thing, but it wasn't Yossarian being confused about it. He used the names alternately when signing soldier's censored letters. IIRC he censored out the signature after having blacked out everything but the articles, or all the nouns, or all the verbs, or some other arbitrary choice of words.