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 next sublevel up is the 2p. Since it has l=1 and thus three orbitals, it can hold six electrons. Filling it leaves us with two electrons. The next level is the 3s, which can hold two electrons. All electrons have now been placed, and we can write the electron configuration of the Mg atom as.