What does the author mean by this statement? America was founded not necessarily on a propositionwritten by Thomas Jefferson in 1776, but on an argument about what that proposition means

I've pasted the section in which this quote is found below.

"The source of the disagreement goes much deeper, however, involving conflicting attitudes toward government itself, competing versions of citizenship, differing postures toward the twin goals of freedom and equality. But the key point is that the debate was not resolved so much as built into the fabric of our national identity. If that means the United States is founded on a contradiction, then so be it. With that one bloody exception, we have been living with it successfully for over two hundred years. Lincoln once said that America was founded on a proposition that was written by Jefferson in 1776. We are really founded on an argument about what that proposition means."

What we have here is Ellis' opinion that the Founders were vague enough or left room enough for the constitution to be interpreted........ namely Jefferson. Jefferson formed a document that has held up since the country's inception for one reason; it is exceptional. Over the years, it has been necessary to tweak (amend) certain things, but on the whole, it is a peice of genious. What you're missing in your question above is that the author didn't write this quote...... it was Lincoln's, and it came at the time of the Civil War. Jefferson claimed all men were created equal, and yet the practice of slavery continued..... thus the leaders of that time were once again forced to debate the validity and meaning of that phrase......