Every year the public reading of Declaration of Independence is re-enacted outside Independence Hall: "That among these are life, liberty and the pursuit of happiness."
Yet, the more familiar phrase -at the time was- "life, liberty and property." So why did Thomas Jefferson replace "property" with "happiness."
Well it turns out Jefferson considered himself an Epicurean - a follower of the Greek Philosopher Epicurus who believed that "pleasure" was life's most worthy goal, although not in quite the way most of us think.
We think of an "epicure" as someone who simply indulges the senses. But Epicurus himself was a celibate vegetarian who defined "pleasure" as living simply, avoiding pain, and when bad things do happen, accepting it.
He believed that the gods cared little for human affairs and that friendship was more important than worship. That's very deep, but it might help explain what gave a man like Jefferson the courage to put his name to a document that at the time was an admission of treason.
As an Epicurean, Jefferson's philosophy would have been:
Don't fear god,
Don't worry about death;
What is good - is easy to get, and
What is terrible - is easy to endure.
Now as to whether he himself achieved happiness is hard to say. After his wife died he never remarried. He had to deal with the contradiction of believing "all men are created equal" while owning slaves, and he died so deeply in debt that instead of freeing his slaves, his surviving family had to auction them off.
But that's probably why he wrote "pursuit" of happiness.