Even ancient scholars did not know the meaning. Plutarch, the famous Greco-Roman author, became a priest at Delphi and wrote an essay with an overview of five theories. Other scholars, over the centuries, have also put forth their theories.
So it might be a bit rash of me, a high school history teacher in 2018, to reject their solutions and propose my own.
The main issue with the previous solutions is the translation and interpretation of the third maxim, or warning. Visitors to the temple were greeted by three warnings (which were carved on a pillar, into a wall, or hanging in the entranceway, depending on the historical source).
The first two warnings are famous:
“Nothing in Excess!”
These are translated in various ways, but the underlying meaning is clear. However, the third warning has two translations that do not seem consistent:
“Make a pledge and mischief is nigh!” or
“Give surety, get ruin!”
Almost every available source has some version of these two; the sources then wax poetic. What is missing from all of these translations is cultural context:
Before the reforms of Solon, the renowned Athenian leader, there was a system in which you could borrow money and offer yourself as collateral. Failure to repay the loan would result in your enslavement.
The warning is thus very practical and philosophical (like the first two). If you are so desperate as to offer your freedom as collateral (pledge or surety), you are guaranteed to become a slave. Or more laconically,
“Bet your freedom, guarantee your slavery!”
It is a bracing formulation of the value of freedom, so often discussed by ancient philosophers.
So how does this connect to the golden “E?” A Socratic dialogue gives a clue:
In Plato’s Charmides, the character, Critias, that Socrates is questioning, makes an interesting observation:
“‘Know thyself!’ and ‘Be Temperate!’ are the same…” (“Be temperate” is an alternative translation of “Nothing in Excess.”) They go on to discuss the idea that self-knowledge leads to temperance, or the avoidance of excess.
And there, the mystery is broken wide open:
All three Delphic warnings are the same. What would cause someone to offer themselves as collateral? Excess. How do you know yourself? Observing yourself when you are free. How do you avoid excess? Knowing yourself. And on and on.
Or more pithily: self-knowledge leads to self-control which leads to freedom which leads to self-knowledge.
The “E” is not an “E” but an illustration of the three warnings as three horizontal lines and the vital connection between the three as the vertical line.