Why does the Selection Principle imply the Ordering Principle?

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Jech's Axiom of Choice book, exercise 4.12 is that the Selection Principle (for all $\mathcal{F}$ there is a function $f: \mathcal{F} \rightarrow V$ such that $|X| \ge 2 \rightarrow \emptyset \subsetneq f(X) \subsetneq X$) is equivalent to K(1) (every set can be injected into $\mathcal{P}(Ord)$). He remarks, "Consequently, the Selection Principle implies the Ordering Principle [every partial order can be extended to a total order]." This isn't obvious to me.

My guess is the intended proof is something like this: "Let $(P, \le)$ be a partial order. We may assume $P=\mathcal{P}(\alpha),$ for some ordinal $\alpha.$ There is some canonical way to strictly extend the ordering. By transfinite iteration, there is a canonical total extension of the ordering." I can't tell what this canonical way to extend $(P,\le)$ is. My first guess was to find the minimal $\beta<\alpha$ such that two incomparable elements differ at $\beta,$ and for all pairs $p \perp q,$ where $\beta \in q \setminus p,$ declare $p<q.$ Unfortunately, this can create a cycle in the order. I'm not sure what else to try.

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You are misreading things.

The ordering principle states "Every set can be linearly ordered". The order extension principle states "every partial order can be extended to a linear order".

If every set can be injectively mapped into $\mathcal P(\alpha)$ for some ordinal $\alpha$, then it will inherit the linear ordering definable on $\mathcal P(\alpha)$.

Interestingly enough, I cannot find any work separating Kinna–Wagner's Selection Principle (Or $\sf KW$ or $\sf KWP_1$ as it is sometimes denoted) from the Order Extension Principle. Either the non-implication is known, or the question is still open, as indicated by the diagrams in Herrlich's "Axiom of Choice" book.