In Schaefferian theory, Abstract and Concrete are "two isotopes of reality", two faces of every perception, interdependent and complementary, which must be reconciled and balanced in music, against the excess of concrete (in musique concrète) or the excess of abstract (in serial and other types of "a priori" musics).
When in 1948 Pierre Schaeffer gave the name Concrète to the music which he invented, he wanted to demonstrate that this new music started from the concrete sound material, from heard sound, and then sought to abstract musical values from it. And this is the opposite of classical music, which starts from an abstract conception and notation leading to a concrete performance.
Furthermore, Schaeffer’s table in the quatre écoutes is made by crossing horizontally and vertically the two fundamental sets of opposites which are found in every perceptual activity: Objective/Subjective and Abstract/Concrete.
The sound object, the correlate of reduced listening is defined as the synthesis of an abstract goal and a concrete goal which refer back to it (the object), instead of using it to get at an (abstract) meaning or a (concrete) source. (Paraphrase of Michel Chion (1983). Guide des Objets Sonores. Eds. Buchet/Chastel, Paris. 1995 translation by John Dack/Christine North.)
See also:Musique Concrète, Quatre Écoutes