Book 3, Chapter 11: The Death of the Body Politic explained simply

CHAPTER XI THE DEATH OF THE BODY POLITIC Such is the natural and inevitable tendency of the best constituted governments. If Sparta and Rome perished, what State can hope to endure for ever? If we would set up a long-lived form of government, let us not even dream of making it eternal. If we are to succeed, we must not attempt the impossible, or flatter ourselves that we are endowing the work of man with a stability of which human conditions do not permit. The body politic, as well as the human body, begins to die as soon as it is born, and carries in itself the causes of its destruction. But both may have a constitution that is more or less robust and suited to preserve them a longer or a shorter time. The constitution of man is the work of nature; that of the State the work of art. It is not in men's power to prolong their own lives; but it is for them to prolong as much as possible the life of the State, by giving it the best possible constitution. The best constituted State will have an end; but it will end later than any other, unless some unforeseen accident brings about its untimely destruction. The life-principle of the body politic lies in the sovereign authority. The legislative power is the heart of the State; the executive power is its brain, which causes the movement of all the parts. The brain may become paralysed and the individual still live. A man may remain an imbecile and live; but as soon as the heart ceases to perform its functions, the animal is dead. The State subsists by means not of the laws, but of the legislative power. Yesterday's law is not binding to-day; but silence is taken for tacit consent, and the Sovereign is held to confirm incessantly the laws it does not abrogate as it might. All that it has once declared itself to will it wills always, unless it revokes its declaration. Why then is so much respect paid to old laws? For this very reason. We must believe that nothing but the excellence of old acts of will can have preserved them so long: if the Sovereign had not recognised them as throughout salutary, it would have revoked them a thousand times. This is why, so far from growing weak, the laws continually gain new strength in any well constituted State; the precedent of antiquity makes them daily more venerable: while wherever the laws grow weak as they become old, this proves that there is no longer a legislative power, and that the State is dead.