And once again, why is this a big deal? Well, it tells you that if you have any continuous function f, and that's what we assumed. We assumed that f is continuous over the interval. There exists some function. There exists a function. You can just define the function this way as the area under the curve between some end point or the beginning of the interval and some x. If you define a function in that way, the derivative of this function is going to be equal to your continuous function.