And his work contributed to the service, not of the doctrine of the past, but of the doctrine of the future.
For he attempted to extend the Cartesian theory to social facts.

He laid down that political, like physical, phenomena are subject to general laws.

He had already conceived this, his most striking and important idea, when he wrote the Considerations on the Greatness and Decadence of the Romans (1734), in which he attempted to apply it: It is not Fortune who governs the world, as we see from the history of the Romans.

There are general causes, moral or physical, which operate in every monarchy, raise it, maintain it, or overthrow it; all that occurs is subject to these causes; and if a particular cause, like the accidental result of a battle, has ruined a state, there was a general cause which made the downfall of this state ensue from a single battle.
In a word, the principal movement (l'allure principale) draws with it all the particular occurrences.
But if this excludes Fortune it also dispenses with Providence, design, and final causes; and one of the effects of the Considerations which Montesquieu cannot have overlooked was to discredit Bossuet's treatment of history.
The Esprit des lois appeared fourteen years later.

Among books which have exercised a considerable influence on thought few are more disappointing to a modern reader.