A more thorough explanation:

A synthetic rule, also known as a quantitative rule, is an evidentiary rule that requires additional evidence to be presented before a case can be closed. This rule exists because certain types of evidence are known to be weak or unreliable.

For example, hearsay evidence is often considered weak because it is secondhand information. Therefore, a synthetic rule may require that hearsay evidence be accompanied by additional evidence, such as eyewitness testimony or physical evidence, before it can be used to make a decision in a case.

Another example of a synthetic rule is the requirement for corroboration in cases of sexual assault. Because sexual assault cases often involve a lack of physical evidence, a synthetic rule may require that the victim's testimony be corroborated by additional evidence, such as DNA evidence or witness testimony, before a conviction can be made.

Overall, synthetic rules are in place to ensure that decisions are made based on reliable and trustworthy evidence, rather than on evidence that may be weak or unreliable on its own.