If I have the timeline right, he wrote this after an intense period of study in 1791. He dove into political philosophy and history, including Xenophon, Plato, Aristotle, Thucydides, Strabo, Dionysius Halicarnassus, Livy, and Plutarch. His modern sources included Montesquieu, Gibbon.
Anyway, he came away agreeing with Aristotle that the preservation or destruction of the constitution depended on the maintenance and renewal of its principles.