“They began to discuss the problem of the vacuum. Descartes became particularly serious on the subject. The others explained a recent experiment to him and asked him what he thought entered into the space of the emptied tube. He said that it was his ‘subtle matter’. My brother [Blaise Pascal] responded to this theory as best he could. Believing that my brother was having some difficulty expressing himself, Roberval took on Descartes with not a little passion, although he remained civil. Descartes responded rather bitterly that he could speak to my brother as long as he desired because my brother spoke reasonably but that he wouldn't continue to talk with Roberval, because the latter spoke out of too many prejudices. With that, he glanced at his watch and saw that it was noon. He stood up, because he had a dinner date in the Faubourg Saint-Germain.”

— Jacqueline Pascal (1647), “Letter to [Name]”, Sep 25 [3]

“For Descartes, the mercury was sustained by the weight of the atmosphere, but the Torricellian space was filled by some form of subtle matter. For Descartes’ inveterate opponent Roberval, the Torricellian space was indeed empty, but the height of the mercury depended upon the limit of the horror of the vacuum (horror vacui).”

— Steven Shapin (1985), Leviathan and the Air Pump [1]