# What are “instantaneous” rates of change, really?

Let’s take instantaneous speed, for example. If it’s truly instantaneous, then there is no change in $x$ (time), since there’s no time interval.

Thus, in $\frac{f(x+h) - f(x)}{h}$, $h$ should actually be zero (not arbitrarily close to zero, since that would still be an interval) and therefore instantaneous speed is undefined.

If “instantaneous” is just a figure of speech for “very very very small”, then I have two problems with it:

Firstly, well it’s not instantaneous at all in the sense of “at a single moment”.

Secondly, how is “very very very small” conceptually different from “small”? What’s really the difference between considering $1$ second and $10^{-200}$ of a second?

I’ve heard some people talk about “infinitely small” quantities. This doesn’t make any sense to me. In this case, what’s the process by which a number goes from “not infinitely small” to “ok, now you’re infinitely small”?
Where’s the dividing line in degree of smallness beyond which a number is infinitely small?

I understand $\lim_{h \to 0} \frac{f(x+h) - f(x)}{h}$ as the limit of an infinite sequence of ratios, I have no problem with that.

But I thought the point of a limit and infinity in general, is that you never get there.
For example, when people say “the sum of an infinite geometric series”, they really mean “the limit”, since you can’t possibly add infinitely many terms in the arithmetic sense of the word.

So again in this case, since you never get to the limit, $h$ is always some interval, and therefore the rate is not “instantaneous”.
Same problem with integrals actually; how do you add up infinitely many terms? Saying you can add up an infinity or terms implies that infinity is a fixed number.

$$f'(x)=\lim_{h\to0}\frac{f(x+h)-f(x)}hf'(x)=\lim_{h\to0}\frac{f(x+h)-f(x)}h$$
That being said, here’s no reason why you should find it helpful. If it’s too fluffy to be useful for you that’s fine. But you’ll need some intuition on what derivatives are supposed to be describing. I like to think of it as “if I squinted my eyes so hard that $$ff$$ became linear near some point, then $$ff$$ would look like $$f’f'$$ near that point.” Find something that works for you.