\begin{definition}[Definition:Conic Section/Focus-Directrix Property/Historical Note]
The focus-directrix definition of a conic section was first documented by {{AuthorRef|Pappus of Alexandria}}.
It appears in his {{BookLink|Collection|Pappus of Alexandria}}.
As he was scrupulous in documenting his sources, and he gives none for this construction, it can be supposed that it originated with him.
\end{definition}