The famous opening paragraph of the book sets the scene for Dickens's subsequent treatment of paradox. Here, Dickens establishes what is without a doubt the fundamental paradox at the heart of the Enlightenment:
[I]t was the season of light, it was the season of darkness.
The intellectual movement known as the Enlightenment had swept over Europe in the 18th century, overturning old ideas and ways of doing things. The watchword of the Enlightenment was "reason." Reason was the standard against which ideas, institutions, and social practices should be measured. If they failed to measure up to this standard, then they were to be changed according to rational principles. The Enlightenment was hugely influential all over Europe—not to mention the American colonies—but it was especially so in France, and Enlightenment ideas provided the intellectual foundation for the French Revolution.
Paradoxically, however, the French Revolution, in its savagery and violence, harkened back to a much more primitive, less enlightened age. And this is primarily what Dickens is referring to in the opening paragraph. Although this may have been the Age of Reason, the "season of light" as Dickens calls it, it was also marked by widespread "darkness"—by violence, ignorance, and superstition in the form of the quasi-religious cult of the Revolution.