On page 19 of Rudin's Principles of Mathematical Analysis, the author constructs real numbers using Dedekind cuts. The proof that the sum of a real number $x$ and its negation $(-x)$ equals $0$ is somewhat clever and more involved. I don't exactly follow it?
Rudin defines the cut $\beta = -\alpha$ as:
$$\beta := \{p\in\mathbb{Q}:\exists r>0 \text{ such that }(-p-r)\notin \alpha\}$$
In other words, some rational number smaller than $-p$ fails to be in $\alpha$.
We show that $\beta \in \mathbb{R}$ and $\alpha + \beta = 0^{*}$.
If $r \in \alpha$ and $s \in \beta$, then $-s \notin \alpha$. Hence, $r < -s$, so $r + s < 0$. Thus, $\alpha + \beta = 0^{*}$.
Question. Why does $s \in \beta \implies -s \notin a$ mathemtically?
I mentally picture it as follows: