In many ways, the leaders of the movement known as the Enlightenment sought to completely re-examine the way that Europeans thought about long-established social and religious notions. The eighteenth-century French philosopher, Denis Diderot, summed up this attitude when he wrote in his encyclopedia that

All things must be examined, debated, investigated without exception and without regard for anyone's feelings.

First of all, the philosophers of the Enlightenment wanted to show that Europeans were not living in the best of all possible worlds. Instead of relying on the clergy and upper-classes to tell the masses that the old ways were the best ways, these thinkers stressed the importance of individual inquisitiveness and the possibility of improvement. In some ways, this focus on an individual's abilities to improve and think freely was an outgrowth of humanist traditions of the Renaissance.

For instance, Jean-Jacques Rousseau stressed education reform based on personal experience and following one's innate curiosity with his work Emile. Wealthy women also shook up social norms by hosting gatherings, known as salons, in which philosophical ideas could be freely discussed. Enlightenment thinkers were largely interested how people of different social classes should relate to each other and the relationship between citizens and their government. This gave rise to support for democratic institutions (and eventually popular democratic revolutions in America and France) as well as reforms to criminal justice systems.

As far as religious thought was concerned, Enlightenment thinkers stressed reason over blind faith. David Hume, for instance, argued that belief in the divine was little more than superstition. Others, known as deists, took a more measured approach. They argued that God exists but takes no action in worldly affairs. This new approach to religion caused many to rethink the rationale behind the religious violence that had consumed much of Europe during the Reformation.