The execution of a custodial sentence can be modulated by many facilities allowing offenders not serving his entire sentence in custody. The legislator by the law of March 4th, 2002, made possible a suspension of the punishment. It can be applied regardless of the nature and duration of the sentence, on the conditions that the prisoner is affected by a pathology engaging the prognosis for survival or what his health is incompatible with the detention. Jurisprudence in a decision of September 28th, 2005, specified that the prognosis for survival must be engaged in the short term. Following this change led to positive law, it is requested that the medical expert to make a prediction time. Does the medical expert have there the scientific opportunity to answer the question? If the answer is not purely scientific, then what is its legitimacy and is the ethic respected? The analysis of the terms of missions sent to the experts and their response have raised several issues: lexical, epistemological and ontological. The problem of the transposition of prognosis illustrates the complexity, in medical sciences, of validity of knowledge and their interaction with the variability of the living. A common reflection between medical and judicial profession could probably answer in a simple and efficient way these difficulties.