Historically speaking, there was a spectrum of conditions between chattel slavery and freedom: serfdom, peonage, indentured servitude, apprenticeship, etc.
Our Declaration of Independence, oddly, probably made the contrast between freedom and slavery more stark. If all men are created equal, yet some men are slaves, then in some way those slaves aren’t “really” men, are they?
There are three logical ways to address this apparent contradiction:
1. Determine that slavery must be abolished.
2. Decide that slaves, and by extension blacks, aren’t really human in the full sense. This is what Taney did in the Dred Scott decision.
3. Decide that the Founders were just plain wrong. All men AREN;T created equal. This is what Calhoun decided and the notion Stephens incorporated into his Cornerstone Speech.
I also make this argument quite often. Researching the Natural Born Citizen issue led me back to the Declaration of Independence as the source of both US Citizenship, and the trigger for the Abolition movement that swept the US as a result of those words in the Declaration.
The Declaration started the Nation, AND started the Abolition movement here.