“Anything, no matter how bizarre, was cited as the cause for slave rebellions except for the most obvious source: slavery.”

Conclusion
1880s, Personal Memoirs of General U. S. Grant (1885)
Context: The cause of the great War of the Rebellion against the United States will have to be attributed to slavery. For some years before the war began it was a trite saying among some politicians that 'A state half slave and half free cannot exist.' All must become slave or all free, or the state will go down. I took no part myself in any such view of the case at the time, but since the war is over, reviewing the whole question, I have come to the conclusion that the saying is quite true.

1860s, Second State of the Union address (1862)

1860s
Context: I never was an abolitionist, not even what could be called anti-slavery, but I try to judge fairly and honestly and it became patent in my mind early in the rebellion that the North and South could never live at peace with each other except as one nation, and that without slavery. As anxious as I am to see peace established, I would not therefore be willing to see any settlement until the question is forever settled.

Letter to Elihu Washburne (30 August 1863)

Source: Educated (2018), Chapter 37, “Gambling for Redemption” (p. 307)

Þórunn of Kambar
Heimsljós (World Light) (1940), Book Two: The Palace of the Summerland

Søren Kierkegaard, Upbuilding Discourses in Various Spirits, Hong p. 327
1840s, Upbuilding Discourses in Various Spirits (1847)

1860s, Allow the humblest man an equal chance (1860)
Context: These natural and apparently adequate means all failing, what will convince them? This, and this only; cease to call slavery wrong, and join them in calling it right. And this must be done thoroughly — done in acts as well as in words. Silence will not be tolerated — we must place ourselves avowedly with them. Douglas's new sedition law must be enacted and enforced, suppressing all declarations that Slavery is wrong, whether made in politics, in presses, in pulpits, or in private. We must arrest and return their fugitive slaves with greedy pleasure. We must pull down our Free State Constitutions. The whole atmosphere must be disinfected of all taint of opposition to Slavery, before they will cease to believe that all their troubles proceed from us. So long as we call Slavery wrong, whenever a slave runs away they will overlook the obvious fact that he ran because he was oppressed, and declare he was stolen off. Whenever a master cuts his slaves with the lash, and they cry out under it, he will overlook the obvious fact that the negroes cry out because they are hurt, and insist that they were put up to it by some rascally abolitionist.

The Stranger (1942)

Source: The New Science of Strong Materials (or, Why You Don't Fall Through the Floor) (1976), Chapter 7, Glue and Plywood (or, Mice in the gliders)