# What's so different about limits compared to infinitesimals?

If you find the limit is 2 for a given function, wouldn't this be the same as $2 + \epsilon$ with $\epsilon$ being a negligible value? This different way of defining limit-like behavior seems rigorous enough, but it took until Abraham Robinson in the 60s to really define the foundation for nonstandard analysis. My question is: what really is the difference?

• The difference is that I could explain to you exactly what "limit" means, but "negligible value" doesn't mean anything. – Jack M Jun 17 '16 at 20:34
• I imagine the problem arises when you have a "negligible value" divided by a "negligible value" yielding a real number i.e. a derivative, or adding an infinite number of "negligible values" together to again yield a real number i.e. an integral. This needs to be well defined, so we need rigorous definitions. – JasonM Jun 17 '16 at 20:38
• @JackM thanks for your answer, but how exactly would a limit be different from a negligible value? I get epsilon-delta and that is rigorously defined, but we're essentially taking the standard part function of the infinitesimal to obtain the exact value of the limit, from my understanding. And thank you Jason, that does make sense. I just don't really understand how limits are very much different. The error terms become infinitesimal in an infinite Riemann sum, but we don't treat them as infinitesimal, we just say they are zero. That is the only difference I see. – rb612 Jun 17 '16 at 20:42
• The difference is that $\varepsilon,\delta$ are real numbers, but infinitesimals are not real numbers with the exception of $0$. Then you need a new axiomatic set theory for this, to handle hyperreal numbers properly. This is a lot more than work and sophistication to something that can be explained easily just using real numbers. – Masacroso Jun 17 '16 at 20:59
• @AndréNicolas, you need the axiom of choice even for such an innocuous fact that the Lebesgue measure is countably additive. Sometimes we see a double standard at work in applying the AC criticism. – Mikhail Katz Jun 19 '16 at 8:32