The Enlightenment foregrounded rationalism and empiricism as the basis of knowledge, which most modern people have seen as an advance over superstition and unquestioned obedience to received authority. There is a comfort in hypotheses that can be tested and verified (or not), in experiments that can be duplicated. Most of us also could not live without the technological advances that the primacy of science brought, such as electrical lighting, mass transit, vaccines, phones, and computers.

The Enlightenment led to ideas such as the natural rights of man which posited that certain rights, such as individual liberty, were part of the nature of the universe and not dependent on the whims of a monarch or government. This resulted in many developments we find positive, such as the implementation of democratic governments. 

However, as many noted after World War I and World War II, the same rationalism and faith in science that improved life immensely during the 19th century also led to the killing technologies that made modern warfare such a lethal—and even species-threatening—proposal. Almost no one is excited by the idea of nuclear holocaust destroying the planet. In the last 40 years, we also discovered how carbon dioxide put into the atmosphere by modern technology causes global warming and climate change, another threat to life on the planet. The Enlightenment ideas that we can control and manage nature without consequence have increasingly been critiqued. 

Theorists such as Edward Said have criticized Enlightenment methodologies, such as the French under Napoleon mapping every bit of Egypt, for facilitating imperialism. 

Many have responded to the excess rationalism associated with the Enlightenment;  the rise of Hitler's spectral, irrational Nazi movement with its eerie torchlight processions and emphasis on violence has been cited as a  backlash movement in opposition to the Enlightenment. Nazis, including Hitler, criticized Enlightenment thought— which they associated with the French—as a force suppressing the zeitgeist and primal force of the German people.  Writers like Aldous Huxley have critiqued and parodied the idea of technology arising from Enlightenment rationalism creating a sterile and pain-free environment for humans.

Many of the ideas that emerged from the Enlightenment have become so central to modern Western thought that the movement has become associated with modernity itself. The foundations of modern liberalism, including pluralism, representative government, and protection of civil liberties, found their first real expression in the Enlightenment, through the thought of men like Locke, Montesquieu, Voltaire, and Beccaria. Additionally, the Enlightenment represented the emergence of a critical, rational worldview that subjected time-honored institutions and practices to scrutiny, providing an intellectual framework for social reforms. So, in highlighting the "pros" of the Enlightenment, we can find in the movement many of the intellectual foundations for what we view as good about modernity.

On the other hand, in searching for "cons," we can find the origins of what we might view as negative aspects of modernity. Even during the Enlightenment itself, many intellectuals, particularly in Germany, chafed against what they saw as the excessive rationalism of the movement. They found the mainstream of Enlightenment thought to be stifling and sterile, and their critique of this aspect of the movement would resonate with nineteenth-century Romantics. Others were troubled by the idea that human institutions could be reinvented, solely with reason as a guiding principle. Jonathan Swift and Edmund Burke, though separated by more than half a century, each expressed this concern. Finally, some modern critics have seen the Enlightenment as a force for order that borders on the tyrannical. Michel Foucault, for instance, has described the effects of the Enlightenment on categorizing and normalizing human behavior in a highly negative light. From a historiographical standpoint, modern historians have even begun to challenge the idea that a unified movement that can be described as the Enlightenment even existed in the first place.