It’s a perennial puzzle: How could the author of the Declaration of Independence, with its soaring proclamation of human equality, justify in his own mind remaining a slave owner?

Thomas Jefferson “never thought that slavery was morally justifiable,” write historians Helo and Onuf, of the University of Helsinki and the University of Virginia, respectively. But neither did he think that he had violated “the natural rights of man” by having been born into a slaveholding family.

Jefferson’s thinking was grounded in a complicated but coherent “historical conception of morality.” Slavery was as old as Western civilization, and even the great liberal philosopher John Locke (1632–1704) had argued that victors in a just war were morally justified in enslaving (rather than killing) their captives. No longer, Jefferson insisted. The “moral sense” had shown a further “remarkable instance of improvement.”

But that was not to say slavery needed to end immediately. Long before the American Revolution, white Virginians, in Jefferson’s view, “had developed institutions of government and made laws for themselves and so had emerged as a distinct people with a civic and moral identity.” Until the enslaved blacks did the same, they had no rights.

Jefferson’s “primary goal was not to free black people,” observe Helo and Onuf, “but to free white people from the moral evil of being slaveholders.” (In his draft of the Declaration of Independence, Jefferson accused Britain of having imposed the institution of slavery on the colonies, but the congressional editors of the draft excised the charge.) The challenge “was to find a practical solution to the slavery problem that would enable Virginians collectively to extricate themselves from the institution, reversing the process of historical development that had deprived Africans of their freedom, but doing so in a way that would not jeopardize the free institutions that were themselves the products of history.”

“Jefferson’s solution to the slavery problem was to institute a program of gradual emancipation, separate slave children from their parents in order to prepare them for freedom,” and create a new state in Africa. Jefferson didn’t do much to advance the cause, and he emancipated only a few of his own slaves, but he believed that Virginia’s slaves would one day be free.