Why does lincoln call the idea that "all men are created equal" a "proposition," rather than a self-evident truth as in the declaration of independence?

The difference between “holding” equality as a “self-evident truth” and regarding it as a “proposition” is that when you understand a truth you have no choice but to “hold”, to value that belief where a proposition you must prove it to be true through demonstration and dedication, like they did with the war

they believed democracy was worth the fight

d: their own individual powers within the new goverment