QuestionsWhat is a just society?A prince must be a philosopher
What is a just society?Virtue and harmony

A prince must be a philosopher

Erasmus 1466 · 1536In Praise of Folly: learning against fanaticism.
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What it claims

Unless you are a philosopher, you cannot be a prince; a tyrant, yes.

Why

A king rules for everyone's good, a tyrant for himself; whoever mistakes false goods for true ones cannot aim at the common good.

In short

"A prince must be a philosopher" is an argument by Erasmus (The Education of a Christian Prince (1516), chapter 1) on the question "What is a just society?" (answer: virtue and harmony). Its claim: Unless you are a philosopher, you cannot be a prince; a tyrant, yes. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P3). AskPhilosophy presents 2 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

The Education of a Christian Prince (1516), chapter 1
Valid with an implicit premise

In 1516 Erasmus wrote a handbook for the future Charles V, then 16. He takes up Plato's idea: cities will be happy only if kings become philosophers. But he specifies what he means by "philosopher": not an expert in logic, but someone who sees through false goods (glory, wealth, flattery) and pursues the true ones. And since the people imitate their prince, a good prince makes a whole country better.

P1What distinguishes the king from the tyrant is not the title but the aim: the king rules for the good of all, the tyrant for himself.
P2Being a philosopher means seeing through the false images of things and pursuing true goods. Whoever is not a philosopher in this sense takes glory, money or flattery for true goods.
P3 · implicit premiseWhoever takes false goods for true ones does not rule for the good of all, but for his own desires and those of his flatterers.
Therefore, unless you are a philosopher, you cannot be a prince; a tyrant, yes.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P3, the computer finds a prince who is not a philosopher, who does not see true goods, and who nonetheless rules for the good of all: so he is not a tyrant.
With P3 the conclusion follows: not a philosopher, so no true goods in view (P2), so no rule for all (P3), so a tyrant (P1). Without P3, one could imagine a prince without wisdom who still rules for the good of all, out of enlightened self-interest, prudence, or because good institutions oblige him to. That is exactly what Machiavelli, and later the thinkers of institutions (Montesquieu), would say: better to rely on good rules than on a virtuous prince.
Analogy. A captain who takes a mirage for an island will steer his ship onto the rocks, even in the finest uniform. It is not the title of captain that saves the crew, but seeing truly.

Objections

2

Each objection is a conversation: click to see who replies and where the debate stands.

Machiavelli (The Prince, 15) 1532 · attacks premise 3

Written in 1513, but printed only in 1532, sixteen years after the Education of a Christian Prince. Many have imagined principalities that never existed. How we live is so far from how we ought to live that a prince who wants to be good in everything comes to ruin among so many who are not. To serve the State, he must learn to be able not to be good, as necessity requires.

Érasme replies
Érasme Erasmus's answer in advance (1516), to a courtier who already makes this reproach
"You are shaping a philosopher for us, not a prince." No, Erasmus answers, it is indeed a prince that I am shaping: what you want is a good-for-nothing like yourself.
The debate goes on (our summary) Erasmus's answer is aimed at a courtier, not at Machiavelli, whom he can hardly have read in 1516 (The Prince circulated only in manuscript, in Florence): it asserts more than it refutes. Machiavelli does not say the prince must be bad, but that he must know how to be when the State is in danger. Their disagreement is about human nature: Erasmus believes in education and example, Machiavelli starts from people as they are.
Machiavelli (The Prince, 17) 1532 · attacks the conclusion

The philosopher-prince wants to be loved. But men love as they please and fear as the prince pleases. If one must choose, it is much safer to be feared than loved: a prince who is too gentle loses his State.

Érasme replies
Érasme Erasmus's answer in advance (1516, before the Prince was published)
A reign founded on virtue and goodwill is more peaceful and more pleasant, but also more lasting and more stable: history shows it. No tyranny, however well protected, has lasted long. He who is feared by all must fear many, and he whose death most men desire cannot be safe.
The debate goes on (our summary) Machiavelli had foreseen this answer: he advises being feared without being hated, by respecting the subjects' property and women. Hatred, for him too, ruins a prince. The question becomes: is fear without hatred possible, and safer than love?