Gunther Schuller's Comment on Louis' introduction to West End Blues from EARLY JAZZ: ITS ROOTS AND EARLY DEVELOPMENT, (Oxford University Press, 1986). Page 116:
"Louis' s West End Blues introduction consists of only two phrases. As has already been intimated, these two phrases alone almost summarize Louis's entire style and his contribution to jazz language. The first phrase startles us with the powerful thrust and punch of its first four notes. We are immediately aware of their terrific swing, despite the fact that these four notes occur on the beat, that is, are not syncopated, and no rhythmic frame of reference is set (the solo being unaccompanied). These four notes should be heard by all people who do not understand the difference between jazz and other music, or those who question the uniqueness of the element of swing. These notes as played by Louis - not as they appear in notation - are as instructive a lesson in what constitutes swing as jazz has to offer. The way Louis attacks each note, the quality and exact duration of each pitch, the manner in which he releases the note, and the subsequent split second silence before the next note - in other words, the entire acoustical pattern - present in capsule form all the essential characteristics of jazz inflection."
Or as Louis himself put it: "Jazz? You'll know it when you hear it."