If you had a credible record of it, sure.
But, you missed my point. You posted to Ruth that you 'knew' that Lincoln 'never' said the 'bringing the South to her knees' line.
So, whereas Ruth may not be able to prove the quote, you can only accurately state that there is not a record of Lincoln saying this but the only proof you could possibly muster that he 'never' said those words is if you were with Abe 24/7 and heard every word that he uttered during this period.
Get it?
For normal people, we recognize that proving someone didn't say something is very difficult.
Because of that, we normally put the burden of proof on the person who claims that a person did say or do something.
And great claims require great evidence.
Fair enough. Now, do you hold Ms. Schwerin to a similar level of documentation? In other words, are you as willing to call bull on her Lincoln quote as mine?
Lincoln is by far the most studied American president. There are loads of sources for his utterances. There are also a lot of people combing through that material for "smoking guns," ranging from evidence that he was gay to evidence that he was a marxist. Most of all, though, there are the Lost Causers, trying to impugn his motives in the war. Those guys have done a lot of research and they have their "smoking guns" (which usually turn out to be taken dreadfully out of context). There's DiLorenzo, for one, and the Kennedys for another. And never once has one of these people come up with a gun so smoking as the one Ms. Schwerin has presented, with absolutely no support.
In other words, if there was such a Lincoln quote out there, it would be all over the place given the number of people looking for such material.
Now, I suppose its possible that Lincoln could have said such a thing, but I feel almost as comfortable in saying that Lincoln never said he wanted to "bring the south to her knees" as I am in saying that he never said, "I like to dress up like a woman and pick up sailors."