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In general, a party seeking to establish a case in court must offer Evidence to support all relevant assertions. However, certain facts are sufficiently well established in courtroom practice that they don't need specific proof. For example, it does not have to be proved that if a driver had a breath-test for alchohol at the scene of a road accident, and the test was positive, that the driver had consumed too much alchohol to drive. In other words, there is a presumption that these devices measure accurately and that the measurements are universally applicable. Similar reasoning applies, for example, to speed measuring devices. In such a case the burden of proof would be on the accused to show that in that specific case the presumption could not be justified. Formal admission Proof Probative presumption of law Probative presumption of fact