What are the requirements for a plaintiff to bring a successful lawsuit against a defendant?
A cause of action is a right to bring a lawsuit. A person or entity may have a cause of action limited by the time frame set forth by statute for bringing a lawsuit. A cause of action may exist under the common law (judge made law) or granted by statutes. In order to have a valid cause of action, all the legally defined elements of a claim must exist. The basis for the cause of action (tort, fraud, breach of contract, etc) must be pleaded in the initial complaint in a lawsuit. The legal theory for the cause of action must also provide for a remedy. Not all injuries have a legal remedy, and therefore, are not a cause of action.
The answer to your question depends on the type of lawsuit to which you are referring. There are different elements of every cause of action that must be proved in order for a plaintiff to bring a successful lawsuit against a defendant and different courts hear different type of cases.