The themes of this work include plenty of comparisons, contrasts, or doubles. We see this in the comparisons of England and France, two major countries. We also see the theme of doubles in the look-alike characters of Charles Darnay and Sydney Carton. There is also the contrast between Charles and his uncle (who was his father's twin), the Marquis, whose personalities couldn't be any more different.
The introduction to the titular "two cities" is also a paradox, contradiction, or reflection of itself: “It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity, it was the season of Light, it was the season of Darkness . . . ”
The first paragraph of the chapter explains, however, the real reason for Dickens's paradoxes: they set the tone for the historical period (the book takes place beginning in 1775). The first chapter compares and contrasts the two monarchies of England and France, along with each country's set of problems. Moreover, the different social structures of these two countries and their faults are described, so that a reader from a number of different backgrounds might relate to or be engaged by this storyline.
Some have suggested that Dickens wanted his work to have mass appeal. Indeed, such contrasts would engage a wider audience than would one set of descriptions.
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.
Dickens uses paradox to establish one of the main themes of the novel. The theme is that during every age, people experience the same struggles, difficulties, and joys. Dickens uses this theme to create universal appeal for the audience. He talks about every age having "wisdom" and "foolishness", "light" and "darkness", "hope" and "despair".
The paradox also sets up the plot of the story which bounces back and forth between England and France. To do this, he discusses the kings and queens of both, the nobility, and the struggles of the common people - again highlighting the universality of it all.