If the Enlightenment preached liberty, why did the Age of Reason not end slavery?

Because the age's ideas about liberty were applied unevenly, and because slavery was built into the colonial economies of the time. Slavery frequently showed the limitations of Enlightenment ideology as it applied to European colonialism, since many colonies operated on plantation economies fuelled by slave labour. The rights of women and non-white people were generally overlooked, and scientific racism first emerged in this period.

Thinkers were not of one mind. Montesquieu argued in The Spirit of Law that slavery was inherently wrong because all humans are born equal, though he thought it perhaps justifiable in climates of intense heat, and he listed arguments for slavery satirically. In Candide, Voltaire has his hero horrified to learn "at what price we eat sugar in Europe". Jaucourt attacked the slave trade in an article for the Encyclopédie, though other authors in the same work reported slavery matter-of-factly or even defended it.

Others have records that historians still debate. Locke wrote against slavery in general but was briefly an investor in the slave-trading Royal Africa Company. A 1766 letter shows Hume passing on a proposal to buy slave plantations in Grenada, though scholars disagree about what it shows. Kant was one of the most notable Enlightenment thinkers to defend racism, although Pauline Kleingeld argues he changed his mind late in life.

The contradiction was noticed at the time. In 1776 the abolitionist Thomas Day mocked American patriots who signed resolutions of independence with one hand while brandishing a whip over slaves with the other. In 1791 the Haitian Revolution began, a rebellion by emancipated slaves against French colonial rule, and European nations and the United States refused to support it despite their Enlightenment ideals.