The foundations of geometry
《The foundations of geometry》 · 第 330 页 · Chapter 14，Section 14.4「Separable Extensions; and Simple Extensions Once Again」，Theorem 14.24「Primitive element theorem」
Finite separable extensions are simple. That is, every finite separable extension $k\subseteq F$ has a ‘primitive element’ $\alpha\in F$ such that $F=k(\alpha)$.