We know that there was a written intent that it was "To promote the progress of science and useful arts". However, who knows whether or not that was the true intent of all those who signed off on it. We see that lots of written intent, (Exhibit A: Purdue's "Partners Against Pain" Oxycontin promotion), may not match the mathematical reality on the ground. Also, we know that there was plenty of places in the Constitution that were good to amend (the three fifths clause, for instance).
This site (http://www.copyrighthistory.org/cam/index.php) has lots of fascinating old docs where you can come up with your own impressions about the early days of copyright. My general impression was that while it didn't ever actually promote the progress of science and useful arts, it absolutely did in the early days serve as a super smart free hack for the new federal government to build a central intelligence and library of all the latest and greatest inventions from throughout the land.
> What does that even mean?
It means that if you analyze it using logic and put all assumptions on the table (start high up on the tree), you deduce that this is a system of intellectual slavery, not of intellectual "property". You deduce that if there is such a thing as stealing ideas, then all ideas with any value are majority stolen and but a fraction novel.
What does that even mean? The intent from the beginning of copyright was to allow people to live off of intellectual works by claiming legal rights over the work.
There are no “first principles” from which basically any societal agreements like these are derived.
Even something as simple as “murder is illegal” isn’t actually derived from any first principles because the government is allowed to murder people, citizens are during self defense, etc.