Ever notice how old math books just straight-up ROASTED their readers? This 1910 calculus book is like "Hey dummy, let me save you from your own terror!" and then explains integrals with such beautiful simplicity that it makes modern textbooks look like they're deliberately trying to confuse you.
The author basically says: "d just means 'a little bit of' and ∫ is just 'the sum of'... that's it! Even 'any fool' can understand this!" (Looking at myself in the mirror: "I am that fool.")
Why did we abandon this glorious approach where calculus was explained like you're a normal human instead of requiring a PhD to understand the explanation of why you need a PhD?