| Ce texte est le contenu de la conférence donnée en Mai 2005 au colloque international La preuve en mathématiques , organisé par P. Cassou-Nogues, P. Mancosu, et M. Panza. A côté de la version anglaise (très imparfaite sans nul doute) est proposée une traduction en français par Olivia Chevalier. | This is the paper given for the conference Proof in Mathematics, organized by P. Cassou-Nogues, P. Mancosu and M. Panza, which took place in Lille in May 2005. It should not be quoted and has to be considered as a draft. The author has to apologize especially for the probably numerous English mistakes. The french version is due to Olivia Chevalier. |