Baudelaire is deeply fascinated by the figure of the dandy, whom he sees as epitomizing the modern attitude in all its finery. At the time when Baudelaire was writing, French cultural life was in the process of rapid change. The dominant Neoclassical aesthetic, which looked to the artistic works of Ancient Greece and Rome as providing the exemplification of beauty, was under attack from Romanticism, with its emphasis on the imagination as the true source of what was beautiful and sublime.
For Baudelaire, the figure of the dandy, with his unique sense of style, epitomized the Romantic spirit. Far from copying the past, the dandy strives to realize and personify the ideal of beauty through his own self-fashioned image. In short, the dandy is both artist and artwork, creating in himself a true work of art. Effectively, the dandy's richly-attired body is the canvas on which he paints an image of beauty.
Most Romantic artists weren't dandies, to be sure, but they did create works out of themselves, so to speak. They may not have gone around wearing outrageously colorful and fashionable clothes but they adopted the same aesthetic attitude to that of the dandy.
The painter of modern life, argues Baudelaire, captures something essential and permanent in the midst of constant flux. And the dandy, as the epitome of modern man, does much the same thing in the way he conducts himself. To be sure, the dandy isn't a fop, a shallow fellow obsessed with the latest fads and fashions. He is a transhistorical and transcultural figure; that is to say he is a universal figure who transcends the restrictive categories of history and nationality.
If one goes all the way back to Ancient Greece one can find dandies in the form of Alcibiades, the notorious adventurer and pupil of Socrates. Then, in ancient Rome, there was Julius Caesar, a man who epitomized the spirit of dandyism, which Baudelaire calls a "mysterious institution." In all cases, the dandy is a man who embodies in his dress and the way in which he comports himself in society both what is fleeting and what is of lasting value. That is what makes him such an important figure, and what makes him, for Baudelaire, the epitome of modern life.