Naked we come into the world, naked we leave it. But in the meantime we do own our bodies. Parents don't, spouses don't, the State doesn't. From hair to toenails, these marvellous organisms are our sole inalienable property. So where state authority is concerned, lines need to be drawn carefully. Two examples lie before us right now.

The first is the Prime Minister's enthusiasm for a new law on transplant organs. It would enshrine the presumption of consent: unless you previously opted out, doctors could assume that after brain death they could use heart, kidney or whatever without consulting relatives. Since only one fifth of us are on the donor register, this would save time and lives. A BBC poll in 2005 attracted a cautious but convincing 61 per cent of public support, and several other countries do it. In Spain there is a “soft” system where relatives are asked even if the patient didn't opt out; in Austria, a “hard” system where relatives can't refuse. In the US there is “required request”, where it is illegal to turn off life-support without asking about transplantation. This prevents the loss of potential organs that occurs when nobody is brave enough to raise the subject with a distraught family.

Some commentators have accused Mr Brown of going for cheap popularity here; I am not among them. He knows what it is to have a child in danger, and what it is to lose one. I think he cares. My main caveat is that with presumed consent the opt-out should be staringly visible. It should be offered in a way nobody could fail to notice, and cost no time, stamps, visits or call centres. Perhaps a tickbox at 16 when you get your national insurance card; then every year a renewable consent box, maybe on your tax form (though given the Revenue & Customs' inability to handle data responsibly, perhaps not). But the opt-out must be unavoidable, universal, not in the small print.

For people have a perfect right to oppose transplantation, and to know clearly that at the moment of removal the brain may be irrevocably dead but the body itself has to be warm, with blood circulating. If that bothers you, that's your right. Others may be spooked by the creepy exhibitionists who write books claiming the existence of “cellular memory” - claiming that after a heart transplant they found themselves wanting the donor's favourite foods or music.

In any legal change, it must be acknowledged and accepted that some of our compatriots have powerfully superstitious beliefs about bodily parts: we are not historically far from the age of relics, and some of the Alder Hey parents held repeated funerals for recovered microscope slides. You may not think that way, I certainly don't; but nobody has the right to gainsay those who do. Not in the “public interest”, not using state authority. Your body is your own.

On the other issue of physicality and authority I find myself on quite the other side. Lord Alton of Liverpool raised last week the importance of providing every child with an honest record of its parentage, whether they were naturally conceived or by donor. One reason is to avoid accidental incest, where the offspring of donated sperm or ova meet and are unknowingly attracted. Adopted children now have the right to know their genetic parents: donor children have no such guarantee. Lord Alton and others say that the “right to lineage” means that on a full birth certificate there should not be lies or omissions. The certificate, he suggests, could be in two forms to protect privacy - a short form (probably soon online) for applying for passports or licences, and a full one for the individual only.

I find that I am with him all the way. For many years, sperm donors often lightly regarded their role as providing a sort of baking soda, to help some unknown woman get a bun in her oven. With the new understanding of DNA, the ability of women to donate eggs, and an increasing popular interest in genealogy, attitudes have distinctly changed. Sperm donors lost anonymity in 2005, despite protests. Yet still families are able to conceal the very fact of donation from their children, leaving it off the birth certificate and not telling them.

This happens, don't doubt it: whether out of embarrassment or possessive affection, sometimes the information is not given to the child early, and it becomes increasingly difficult to raise as he or she grows older. However well meant, this is a serious breach of a child's rights. Phil Willis, MP, chair of the parliamentary committee on the subject, puts it with brutal directness: “If parents want to deceive their children, that's their decision. But it is our view that the State should not be complicit in that.”

If, as some families will angrily say, “We're the loving parents, it isn't important”, then why hide it? I have heard it argued, by one father in this position, that it would make children dream and fantasise destructively about their biological parent - but every child does a bit of this anyway. The foundling fantasy is common- place. At 4, I used to go on about my “real mother and father” who lived down the garden with a character called the Wicked Step-Driver. Tell a child early, in fairytale terms, and it should not be difficult. Hide it and you violate a basic right. And to forestall the other old-school protest - yes, OK, women have indeed told lies about paternity for centuries. That doesn't make it honest.

So I find myself with Lord Alton in saying the State should enforce this honesty on families, even against their inclination; but wary of statist intervention when it comes to hospitals whipping your vital organs out without being absolutely sure you wouldn't mind. But in both cases it affirms that central principle: it's your body, nobody else's.

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