As expert opinion influences judicial decision-making, it is necessary to look into its substratum: scientific proof This proof helps to establish the reality of a fact. Science generates exact and thorough knowledge based on objective circumstances that can be verified. The expert, a man of truth, is solicited by the judge, a man of justice. The quality of a scientific proof depends on the scientific reasoning that led to it. The intellectual process used in the decision-making process has greatly evolved through the centuries, from analogical reasoning to inductive reasoning based on probability. Uncertainty is indissociable from science. The fallibility of expert opinion is the consequence of this assumption. Means allowing scientific truth to lead to judicial truth should be considered. The difference between the standard of quasi-infallibility, required by law and tribunals, and scientific uncertainty, can sometimes be very large. The work of an expert requires intellectual honesty and humility