We have already seen that the derivative of a function $f$ at $x=a$ is $$f'(a)
= \lim_{w \to a}\frac{f(w)-f(a)}{w-a} = \lim_{h \to 0}
\frac{f(a+h)-f(a)}{h}.$$ We can apply the same idea to get a formula
for the derivative of $f$ at every point:$$f'(\Box) = \lim_{h \to 0}
\frac{f(\Box+h)-f(\Box)}{h},$$
where $\Box$ stands for "anything you like".
In particular, if we take $x$ as input and associate to it $f'(x)$ as output, then we have a function from the set of $x$-values at which we can compute $f'$ to the real numbers.
$$
x\mapsto f'(x)$$