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.
We also have to know how many electrons can fit into each state. There is only one s-orbital per shell, and it can hold a total of 2 electrons. There are three total p-orbitals (ml = -1,0,1) that can hold a total of 6 electrons (because each ml can have an ms of +1/2 or -1/2. There are five d-orbitals (ml = -2,-1,0,1,2) that can hold a total of 10 electrons. Here's an example to help put this in context. We need to write the electron configuration for an oxygen atom. First, go to the periodic table and find how many electrons oxygen has.