Abstract : Our paper comes from the work we did in the Association pour le développement du Génie Cognitif, Nice (GECO) and more precisely in a project named CESAME supported by the Institut Universitaire pour la Formation des Maîtres (IUFM) of Nice. CESAME means: « Construction Expérientielle du Savoir, et Autrui dans les Mathématiques Enseignées » and our aim is to interpret what students say or write about their experience of mathematics and more precisely about their experience of the Necessity in mathematics.