We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.
If one takes that sentence seriously then slavery is unthinkable.
Given that the guy that wrote it, and many of the guys who signed it were all slave owners, I think it is not reasonable to believe that they intended those words to apply to slaves.
If they did, wouldn't they have freed all their own slaves?
No, I think that was just a bit of Jeffersonian mischief, and one which had farther ranging consequences than he envisioned.