Restitution is an obligation imposed by law on ground of justice and equity. Its purpose is to prevent unjust enrichment. Unlike express contracts or contracts implied in fact, restitution does not rest upon the assent of the contracting parties.
The "officious intermeddler doctrine" holds that where a person performs labor for another without the latter's request or implied consent, however beneficial such labor may be, he cannot recover therefor.
An exception is that of emergency aid, where the service is needed to prevent the others' bodily harm and the helper is a doctor and therefor doesn't give the implication of gratuitousness.
The fact that a recipient has obtained a benefit without paying for it does not of itself establish that the recipient has been unjustly enriched.
A valid contract defines the obligations of the parties as to matters within its scope, displacing to that extent any inquiry into unjust enrichment.
There is no liability in restitution for an unrequested benefit voluntarily conferred, unless the circumstances of the transaction justify the claimant's intervention in the absence of contract.
Liability in restitution may not subject an innocent recipient to a forced exchange: in other words, an obligation to pay for a benefit that the recipient should have been free to refuse.