A new notion of deductive system is proposed in order to allow the development of a truly universal theory of fibring. After a brief review of the notions of 2-category, multicategory and polycategory, a deductive system is defined as a 3-multipolymulticategory. Objects are sorts, multimorphsims are language constructors, 2-polycells are judgments, and 3-multicells are derivations. The universality of the proposed concept is discussed. Fibring is introduced as a cocartesian lifting. Along the way, some examples are provided. Finally, pending problems are discussed. Joint ongoing work with Marcelo Coniglio, Cristina Sernadas and Luca Viganò.