What is a friend? A single soul dwelling in two bodies.
This is the origin of the expression a soulmate. According to Aristophanes the first souls were split in two. Humans therefore contained only half a soul and would wander the world looking for the other half of their soul – their soulmate.
The image is of Aspasia, an influential woman in her own right, and a lover and partner of the Athenian statesman Pericles.