The Shield

is for protection. It is called the Aegis. “To be under the Aegis of someone.”

In the centre is the head of Medusa, a gorgon, an attribution of Minerva.

The AEIO designations are a Latin mnemonic:

A and I are the first two vowels of affirm, designating the affirmative proposition.

E and O are the vowels in nego, designating negative propositions.

Euler’s circles are a graphic representation of the AEIO forms.