Epsilon-delta is the formalization introduced by Cauchy, Bolzano and Weirstrass during the 19th Century. It is not the way calculus was invented by Newton and

Epsilon-delta is the formalization introduced by Cauchy, Bolzano and Weirstrass during the 19th Century. It is not the way calculus was invented by Newton and Leibniz. They thought in terms of infinitesimals. However, this approach is relatively hard to formalize. It was only done by Robinson in the 1960s. To see why Cauchy et al had to work on epsilon-delta, take a look at [1], an excellent book. [1] https://www.macalester.edu/aratra/ https://www.macalester.edu/aratra/