In law, in its broadest sense, an inference as to the existence of a fact not known or proved to exist, which inference arises from its logical connection or association with certain other facts which are known or proved. As thus defined, a presumption may be nothing more than a mere inference of fact such as a jury is required to make in rendering a verdict.

Thus, proof of the loss of a vessel in a storm with the other attendant circumstances may create a presumption or inference of the fact of death of a passenger sufficient to justify a jury in finding the death of the passenger as a matter of fact, unless the 'presumptive' proof of the fact of death is rebutted or explained away by proof of other circumstances.

In a narrower and more important sense the term signifies an inference of fact which is required by some positive rule of law to be made from the proof or known existence of certain other facts. Thus, the proof that one has not been seen or heard from by his friends or acquaintances for a considerable period, together with other circumstances, may or may not give rise to the presumption of his death; but if the absence is prolonged for a period of seven years, it is a positive rule of the common law that such absence, when unexplained, shall be deemed presumptive evidence of death.

It is evident that the effect of a presumption of this class is to give a weight or significance to facts actually proved in a given case not warranted by logic and not justifying in the absence of an express rule of law a finding by the court or jury of the existence of the fact presumed. Such presumptions are based upon considerations of convenience and serve a useful purpose in aiding in the proof of facts which it might be impossible to establish by any inference logically flowing from facts actually proved. it is for this reason that presumptions of this class are sometimes called 'presumptions of law'; that is presumptions required by the law, as distinguished from mere logical inferences of fact. In strictness, however, such presumptions are true presumptions of fact. The effect of the presumption is prima facie to establish a fact, which, however, may be rebutted by the proof of other facts inconsistent with the fact presumed.

There is still a third class of presumptions so called, which are not true presumptions at all, but legal fictions. They are in reality rules of substantive law, although stated as presumptions of fact, and consequently they cannot be explained or rebutted. Thus the conclusive 'presumption' that a child under the age of seven has not capacity to commit a crime, or that one is presumed to know the law or the contents of certain public records, is not a presumption, but a positive rule of law which cannot be controverted. Oftentimes such rules of law originated as presumptions of fact, as, for example, the rule that twenty years' use of a right or interest in real estate gives rise to the conclusive presumption that such use is by virtue of a lost grant and is therefore lawful. Originally the presumption was a presumption of fact, which might be rebutted like any other true presumption, but with the sanction of a long line of judicial decisions the presumption became adopted as a rule of substantive law, if there had been twenty years' user of the property which established the lawfulness of the use as a matter of law, and the question as to whether the use of property was by a virtue of a lost grant ceased to be a jury question.

The function of true presumption is primarily to aid a litigant in sustaining the burden of proof cast upon him by the pleadings in a case or by the rules of procedure.

Some of the typical presumptions, in addition to those already mentioned, are: One charged with a crime is presumed to be innocent. Infants between the age of seven and twelve are presumed to be incapable of committing a crime. A state4 of facts proved to exist is presumed to continue to exist. Letters having been proved to have been properly mailed are presumed to have been received. The unexplained failure of a party to a litigation to testify in his own behalf gives rise to the presumption that his testimony would be unfavorable to his case. This rule, however, has no application to one placed on trial upon a criminal charge, as the effect of such a presumption would be to deprive the accused of his legal right not to testify. There are many other presumptions of less frequent use which are founded upon the same general principles. See Burden of Proof, under EVIDENCE. Consult the authorities noted under EVIDENCE; also Lawson, The Law of Presumptive Evidence (2d ed., San Francisco, 1899).