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.
Electrons always fill orbitals in the same order. Each s orbital holds 2 electrons, each set. of p orbitals holds 6 electrons, each set of d orbitals holds 10 electrons, and each set of f. orbitals holds 14 electrons. The order in which orbitals are filled, from first to last, is: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p. Aluminum has 13 electrons.