The law of unjust enrichment, together with contracts and tort, forms the third pillar which describes the law of private obligations. Unjust enrichment can be divided into autonomous unjust enrichment and restitution for wrongdoing. In the former, or true unjust enrichment, the action centres on proof of three elements: an enrichment of the defendant, at the expense of the plaintiff, and the lack of a juristic reason to justify the transfer, or, in English law, the presence of an unjust factor concerning the transfer. It has been said that autonomous unjust enrichment covers every action similar to a mistaken payment of money. In the latter, the action is restitutionary only in the sense that the defendant is required to give up gains made from perpetrating a wrong on the plaintiff.