The aim of this paper is to re-describe Brian Ferneyhough's compositional practice as an arrangement of medium/form relations in the sense of system theorist Niklas Luhmann. The production process of an artwork is understood as a procedure by which ‘a form is changed for another’, or rather: by which a form becomes a medium for further form distinctions, such that the contingency of the beginning is transformed into the necessity of the final work of art. Based on the preliminary ‘large-scale form’, the fundamental steps in the composition of the first movement of the Fourth String Quartet are observed as recursive operations in a self-referentially closed process of gaining form. Despite its formal closure, the final score's potentiality becomes a medium for further performative and perceptive actualizations. The artwork establishes itself on the level of second order observations, as a difference between the possible and the actual: between medium and form.