Such a presumption may legitimately arise from a rule of law or a procedural rule of the court or other adjudicating body which determines how the facts in the case are to be proved, and may be either rebuttable or irrebuttable. An irrebuttable presumption of fact may not be challenged by the defense, and the presumed fact is taken as having been proved. A rebuttable presumption shifts the burden of proof onto the defense, who must collect and present evidence to prove the suspect's innocence, in order to obtain acquittal.[2] Rebuttable presumptions of fact, arising during the course of a trial as a result of specific factual situations (for example that the accused has taken flight),[3] are common; but an opening presumption of guilt based on the mere fact that the suspect has been charged is considered illegitimate in many countries[4] and contrary to international human rights standards.