Legal persons are of two kinds: we humans, known in law as "natural persons", and persons that are legal fictions, such as companies and countries. These fictional people solve legal problems that can arise when natural people act in groups.
A company may have tens of thousands of owners who change when stocks are traded. How should you enter into a contract with this ever-changing group of people? Suppose their side of the contract is not honoured. Whom should you sue if all the owners at the time of signing have sold their shares?
This problem is solved by the legal fiction that a company is a person with the rights and duties specified in contracts.
Similarly, when the Prime Minister signs a treaty with China, he does so on behalf of "the country". The individuals who live in New Zealand and China change all the time. So the treaty must be not between all these people but between the legal fictions New Zealand and China.
Fictional people also help when a role is occupied by a succession of different individuals. It can be useful for a president or an archbishop to have rights and duties that accrue to the position rather than the individual who temporarily occupies it.
It is strange that the Attorney-General finds such legal fictions "pretty strange". And even stranger that he thinks granting legal personality to a river is "no stranger".
Rivers are not changing collections of natural people or positions filled by successive individuals. Nor can rivers be legal persons, whatever Parliament says. What duties might the Whanganui River have? Does it have a duty of care to ensure no one drowns in it? Can the river be sued?
The underlying rationale can be seen in Finlayson's claim that Whanganui iwi will "have a representative speaking for the river [and] the Crown has a representative speaking for the river, and they are focused on addressing many of the problems the river has had over the last 140 years".
People can have problems with rivers, such as drowning in them. But rivers themselves cannot have problems. Whatever happens, rivers don't care.
Rivers have no preferences and they cannot enjoy benefits or suffer harms. So they have no interests that can be represented or transgressed.
Someone may reply that companies and countries have no preferences either. But their shareholders and citizens do. Take natural people out of the equation, and talk of rights, duties and representation is senseless.
What can justify this foray into legislative lunacy? Mr Finlayson appeals to traditional Maori thinking. "In their worldview, 'I am the river and the river is me'," he has said. "Their geographic region is part and parcel of who theyare."
If this worldview were literally true, then the Whanganui River would be a natural person - or, rather, many natural people: namely, all those Maori with whom it is identical. But it isn't literally true. It is simply a way of expressing a feeling towards the river.
A river is not a natural person: our legislators have embarrassed themselves.
Jamie Whyte, former leader of the Act Party, is about to become director of research at the Institute of Economic Affairs in London.
