Baudelaire is uncompromising in his requirement that the modern painter should paint modern subjects. The painter's technique may be influenced by the old masters, but he should not paint landscapes or scenes from classical antiquity, or even (as Baudelaire says too many painters do) contemporary figures in outdated costumes. The key characteristics of modern life, according to Baudelaire, are its dynamism and its transience. Although the modern world still contains forests and fields, they do not change: it is the city, with its surging crowds and fleeting fashions, that captures the true spirit of modernity.
The modern painter should seek out modernity with a penetrating eye for detail. Those seemingly insignificant trends in fashion and cosmetics which only last for a few months should attract his eye and furnish his subject. Baudelaire points out that this spirit animated the great painters of the past, who painted what they saw around them. Although the fashions they painted are now out of date, they are perfectly harmonious within the paintings, because they suit the figures and other details. The modern painter should situate himself within the crowd in a great city and transfer to his canvas the energy that surrounds him. Art consists of two halves, says Baudelaire: the ephemeral and the eternal. The painter's skill and the permanence of the medium provide the eternal element, while the fleeting, restless nature of modernity itself furnishes the ephemeral.