The political ideas that fully came together under the name “liberal” in the early 19th century—the ideas we often now refer to as “classical liberalism”—emerged out of major debates and developments from the late 1600s to the late 1700s. These were part of the broad European intellectual movement of that era that came to be known as “the Enlightenment".

This volume shows how the Enlightenment and the development of liberal ideas were woven together by looking at three defining figures of the era: Baruch Spinoza (writing in the mid-1600s), the Baron de Montesquieu (mid-1700s), and Immanuel Kant (whose career reached its height in the final two decades of the 1700s). Both Spinoza and Kant were concerned with fundamental philosophical questions about what we could know about God, morality, the nature of the world, and humanity’s place in it. Montesquieu wrote almost nothing about such questions, drawing instead from global history and comparative law.

While the Enlightenment is associated with many things, one of them was the struggle to understand morality and human nature through the use of reason rather than relying on religious authority. Another was the attempt to understand political and social orders in ways that would prevent a return to the wars of religion that had divided Europe in the 1500s and the first half of the 1600s. In various ways, Spinoza, Montesquieu, and Kant all argued for religious toleration—for the peaceful coexistence of different organized ways of understanding God within civil governments that didn’t enforce any one of those ways. Their support of freedom of religious thought also made all of them supporters of free inquiry and free speech. The three thinkers likewise shared commitments to the rule of law and to constitutional forms of government that would constrain the discretionary power of any one ruler.

This book does not aim to be a complete history of the Enlightenment. Rather, it is an introduction to three of the most important contributors to it. The Enlightenment partly took shape around their contributions.