if i gave you y = ln x as a function. you can easily calculate its derivative dy / dx = 1 / x. but if you tried to find the derivative at x = - 1, there is no point for the tangent to be there since the function does not exist at that point. It is similar for points of inflection for instance with the function y = 1 / x at x = 0. A point of inflection needs a point ( i.e. x and y values )