Every form of government tends to perish by excess of its basic principle.

Durant’s aphorism frames political decay as an internal, self-generated process: the very principle that gives a regime its legitimacy—liberty in democracy, authority in monarchy, equality in socialism, virtue in aristocracy—can become destructive when pursued without restraint. The line echoes a classical “golden mean” sensibility (and the ancient theory of regime cycles), suggesting that stable government depends on balance, counterweights, and limits rather than ideological purity. It also implies that political failure is often less a matter of external enemies than of a system’s tendency to intensify its own defining impulse until it turns into its opposite (e.g., liberty into license, authority into tyranny).