Manet was close friends with Baudelaire. In many ways his art takes from the inspiration of his friend's take on Modernist philosophy in art, in daily life, and in the higher sense of universal truth. Consequently, Manet's artworks centered heavily around metropolitan scenes of everyday individuals, often complete strangers—because he was being a man of the crowd, as it were, and a flâneur, you see. He was also more concerned with how to paint than with what to paint. This is the reason for his very intentional style of brushwork; his art is inventing a new style of painting, ahead of its time, that would inspire the future generation of artists known as the Impressionists.
Ever since the Italian Renaissance we have seen artists making use of one-point linear perspective (going all the way back to Masaccio's Holy Trinity fresco). Even though the canvas (or wall) on which an image is being painted is flat and two-dimensional, artists can, through visual tricks, generate the illusion that the looker is seeing a three-dimensional space—or at least the concept of three-dimensionality resides there in the painting. If you think about it, it's quite extraordinary, actually; that on a flat canvas a scenic landscape of hundreds of miles can be reproduced. This is done through optical devices of artistic style that deceive the viewer—emphasis on the word "deceive." If people like Charles Baudelaire and Édouard Manet were trying to record truth by observing reality around them as flâneurs, then what role would deception play in their published works? Manet decided that it should play no role and adopted this philosophy to his painting. For if art is done on a flat, two-dimensional canvas, there ought to be no trickery or illusions about its two-dimensionality. For this reason, Manet's artwork often appears flat—since it's a painting; and the artist knows it's a painting; and we know it's a painting; and we've all accepted the fact. Just look at this crude self-portrait by the artist, which appears quite two-dimensional and unfinished, if not wholly indiscernible. While visually it does not appeal to our sense of realism, Manet would suggest this painting's style and construction rings more idealistically true to real-life.
Furthermore, Manet's truthful approach to painting included the stylistic decision to paint busy, public scenes without much clarity. He wanted to recreate things the way the human eye really saw them; and since the industrial pace of life had quickened so prodigiously for the new, Modern Era, things tended to move faster than one could keep up with (at least, one like Manet). People came in and out of shops, tripping around the streets, hardly stopping to think or take in their surroundings; or, if they did stop, everyone else around them was still scurrying to and fro. The world moved at an industrious rhythm now, and no pastoral quietude existed in the energetic metropolis of Modern urban life. Sights, then, came to one blurred from this rapidity of city living or hazy from the smoke of factories and mills. Manet's paintings, therefore, often lack detail or appear hazy and unclear because he intends to produce a different effect in his art: that not just of what the ordinary person would see in the late 19th century city scene but of how one sees it.