The Business Of Slavery

Smithsonian features a history of Jefferson's slave enterprise (h/t nc links):

The critical turning point in Jefferson’s thinking may well have come in 1792. As Jefferson was counting up the agricultural profits and losses of his plantation in a letter to President Washington that year, it occurred to him that there was a phenomenon he had perceived at Monticello but never actually measured. He proceeded to calculate it in a barely legible, scribbled note in the middle of a page, enclosed in brackets. What Jefferson set out clearly for the first time was that he was making a 4 percent profit every year on the birth of black children. The enslaved were yielding him a bonanza, a perpetual human dividend at compound interest. Jefferson wrote, “I allow nothing for losses by death, but, on the contrary, shall presently take credit four per cent. per annum, for their increase over and above keeping up their own numbers.” His plantation was producing inexhaustible human assets. The percentage was predictable.
In another communication from the early 1790s, Jefferson takes the 4 percent formula further and quite bluntly advances the notion that slavery presented an investment strategy for the future. He writes that an acquaintance who had suffered financial reverses “should have been invested in negroes.” He advises that if the friend’s family had any cash left, “every farthing of it [should be] laid out in land and negroes, which besides a present support bring a silent profit of from 5. to 10. per cent in this country by the increase in their value.”
The irony is that Jefferson sent his 4 percent formula to George Washington, who freed his slaves, precisely because slavery had made human beings into money, like “Cattle in the market,” and this disgusted him. Yet Jefferson was right, prescient, about the investment value of slaves. A startling statistic emerged in the 1970s, when economists taking a hardheaded look at slavery found that on the eve of the Civil War, enslaved black people, in the aggregate, formed the second most valuable capital asset in the United States. David Brion Davis sums up their findings: “In 1860, the value of Southern slaves was about three times the amount invested in manufacturing or railroads nationwide.” The only asset more valuable than the black people was the land itself. The formula Jefferson had stumbled upon became the engine not only of Monticello but of the entire slaveholding South and the Northern industries, shippers, banks, insurers and investors who weighed risk against returns and bet on slavery. The words Jefferson used—“their increase”—became magic words.
Jefferson’s 4 percent theorem threatens the comforting notion that he had no real awareness of what he was doing, that he was “stuck” with or “trapped” in slavery, an obsolete, unprofitable, burdensome legacy. The date of Jefferson’s calculation aligns with the waning of his emancipationist fervor. Jefferson began to back away from antislavery just around the time he computed the silent profit of the “peculiar institution.”

The whole article is worth reading. This gives the true basis of slavery.  Like Amish raising horses, it makes a good business model to be able to breed you farm machinery.  The 4% quote lays bare the reason why Jefferson never reached the point where it was completely 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."