The Allegory of the Cave. The dialogue between Socrates and Glaucon is probably fictitious and composed by Plato; whether or not the allegory originated with Socrates, or if Plato is using his mentor as a stand-in for his own idea, is unclear. In the dialogue, Socrates asks Glaucon to imagine a cave, in which prisoners are kept.
The d sublevel consists of five orbitals that can hold a maximum of  two electrons each. The total maximum number of electrons that the  d sublevel can hold is 10 electrons.
The usual explanation is that half-filled or completely filled subshells are particularly stable arrangements of electrons. An example is chromium whose electron configuration is [Ar]4s13d5 with a half-filled d subshell, although Madelung's rule would predict [Ar]4s23d4.