On November 11, 1675, Leibnitz demonstrated integral calculus. He was trying to find the area “under” a function. Doing so was quite a feat, opening the door for many advanced mathematical proofs and concepts, but sadly was drowned at the time in the feud between Newton and Leibnitz.
Newton, while a mathematical and physical genius, was petty and vicious towards competition. He tried to erase or diminish any accomplishments not Made-By-Newton™ in his very broad fields of expertise.
In modern calculus, we use the notation for integrals invented by the German mathematician, rather than the Englishman’s.