In law, a presumption is an "inference of a particular fact". There are two types of presumptions: rebuttable presumptions and irrebuttable presumptions. A rebuttable presumption will either shift the burden of production or the burden of proof ; in short, a fact finder can reject a rebuttable presumption based on other evidence. Conversely, a conclusive/irrebuttable presumption cannot be challenged by contradictory facts or evidence. Sometimes, a presumption must be triggered by a predicate fact—that is, the fact must be found before the presumption applies.