“One Man’s Modus Ponens § Slavery and Phrenology”, 2012-05-01 (; similar):
One man’s modus ponens is another man’s modus tollens is a saying in Western philosophy encapsulating a common response to a logical proof which generalizes the reductio ad absurdum and consists of rejecting a premise based on an implied conclusion. I explain it in more detail, provide examples, and a Bayesian gloss.
George Combe (1788–701858166ya) appears to have made a phrenological argument (that African docility implies the morality of abolition rather than maintaining slavery) in 3 places—a marginal note on a letter from a pro-slavery advocate in 1839185ya; his Notes on the United States of America, 1841183ya; and his System of Phrenology, 1843181ya: