When a case comes to court, each party has the onus (burden) of proving those facts on which they rely in making their claims.
In some special circumstances, the law assists one or other party by shifting the normal onus of proof from one party to the other. This is normally done by creating a 'presumption' of facts in favour of one party, so that the other party then has the onus of disproving the presumed facts. An example of this is when one party to a transaction alleges that it was brought about because of undue influence.