Carlyle’s target was … economists such as John Stuart Mill, who argued that it was institutions, not race, that explained why some nations were rich and others poor. Carlyle attacked Mill … for supporting the emancipation of slaves. It was this fact—that economics assumed that people were basically all the same, and thus all entitled to liberty—that led Carlyle to label economics “the dismal science.”
In keeping with the abolitionists origins of the phrase: