In this text, Valérie Barry attempts to: - build intelligibility of difficulties and successes, in a collective situation of mathematics learning, of children defined by the French education system as pupils with important cognitive disorders. She makes three research hypothesis: 1) a systemic approach of pupils' mental activity promotes the revelation of interactional and complex causalities of learning/teaching phenomena ; 2) translating manifestations of pupils' mental activity into educational needs makes it possible to invest a potential space that supports the educability postulate ; 3) A fractal modeling of educational needs helps the researcher to elucidate these needs, and to build interpretations that promote pupils' learning. The dialectisation of theory and action takes place in a clinical action research, which postulates that it is fundamental to take the subjectivity of the protagonists into consideration when building intelligibility of phenomena. The action research was conducted during the university year 2006-2007, in a Medical and Educational Centre, and concerned four pupils that learned numbers and comparative concepts. The analysed has revealed the validity of the three hypothesis, and potentially efficient/effective teaching methods