Did Enlightenment ideas really cause the American and French revolutions?

Partly, and historians still argue about how much. The link itself is well documented: the Enlightenment has frequently been tied to the American Revolution of 1776 and the French Revolution of 1789. The question is whether the ideas caused the revolutions or only supplied the words.

The American case looks strong on paper. Franklin brought the newest European ideas back to Philadelphia, Jefferson incorporated some Enlightenment ideals into the Declaration of Independence, and Madison did the same in the Constitution of 1787. Political scientist Donald Lutz found that Montesquieu was the most frequently quoted authority on government in pre-revolutionary British America, cited by the founders more than any source except the Bible. Yet even here, historian Garry Wills argued that Jefferson drew on the Scottish Enlightenment rather than Locke.

In France the argument is sharper. Alexis de Tocqueville proposed that the Revolution was the inevitable result of the opposition between the monarchy and the Enlightenment's men of letters. Jonathan Israel claims that the ideas themselves caused the change. But the revolutionary demands also grew from resentment of clergy and aristocratic privilege amid an economic crisis, two expensive wars and years of bad harvests, and these demands were couched in Enlightenment ideals.

A fair summary: the revolutions clearly spoke the Enlightenment's language, but whether that language lit the fire or only named it is still debated.