On the momentous day Lincoln signed the Proclamation, he did the most that he, as President, could do. Slavery had been legal in America since 1619, when the first Jamestown settlers used slaves. But, for the 16th American president, "If slavery is not wrong, nothing is wrong."
Here's what Lincoln knew. Africans had been "imported" (that word was used in contemporary flyers) and were "sold at private sale" for work on plantations in the South. What did "imported" mean? Just as it sounds: Africans were taken from one country (their own) and shipped as "cargo" to another (America).
Africans were taken against their will; placed in detention enclosures (a slave barracoon) while still in Africa; and then jammed into ships to travel to the "New World" as products to be sold at auction. Many captives didn't survive the journey. Those who did faced an oppressive life in a country where they couldn't even keep their own names. (Remember Roots - by Alex Haley - where Kunta Kinte became known as "Toby?")
Once sold to a plantation owner, Africans who had children provided the owner with more free labor. Slaves, including young people, faced life in a land where a "Fugitive Slave Law" made escape a crime not just for runaways but for the people who helped them - and - authorized rewards for those who turned them in. (Scroll down to the last sentence of this flyer where the "owner" of a runaway offers a greater reward than "the law allows.") As Lincoln said:
The 16th President did what even the Founding Fathers had not done. He took the first formal step to eliminate the "right" any man had to "own" another.