A proposal to introduce an "opt-out" system for posthumous organ donation, based on the notion of "presumed consent" in place of the present "opt-in" system is before Holyrood.

This might seem to be a good idea. Many people die for the lack of an organ transplant. Far more people say they would like to donate their organs posthumously than actually get round to officially registering to do so. However, we should treat the proposal with caution.

It seems to embody a rather cavalier attitude towards "consent".With regard to ethics, it matters not merely what we do but why we do it. Public policies cannot be justified solely on the basis of their consequences.

Even if it turned out that an opt-out system provided more organs for transplants than an opt-in one, it might still be morally objectionable.

In various contexts, consent is of crucial ethical importance. It can serve to authorise that which would be morally wrong in its absence. The most obvious example is sex. Sexual intercourse without consent is morally wrong. It is rape.

Consent must be voluntarily and knowingly expressed in order to serve the function of permitting particular actions of other people towards and concerning the consenting person that otherwise would be morally wrong.

Wanting something is not the same as consenting to it. To want to have sex with someone is not the same as giving one's consent to have sex with him. To want to marry someone and to say that one does is not the same thing as becoming, by one's consent, a spouse. It depends on how and when one says "I do".

To express the intention to consent to do something is not the same as consenting to do it, as any jilted bride or groom will know.

Opinion polls are not the same as elections or referendums. Scotland would have left or remained in the UK on the basis of how the people who happened to vote actually voted rather than with regard to what the preferences of all the members of the electorate happened to be on the day.

Similarly, to want to donate one's organs posthumously is not the same as giving one's consent to their retrieval after one's death.

"Presumed consent" is a very troublesome notion.

Imagine a couple who have been married happily for many years. Suppose that the man made sexual advances towards the woman and that she expressed neither an enthusiasm for nor an objection to their continuation.

Can consent be presumed? If consent can reasonably be presumed in this particular sort of case it is, I suggest, only because of actions in the past that were explicit manifestations of consent.

If people explicitly specify that they give their consent to the use of their organs for transplantation when they die, we might reasonably presume that such consent has continued if they die in old age without withdrawing it.

However, if they do not ever explicitly consent to such a use of their organs, it would be outrageous to say that we can reasonably presume that they did consent on the grounds that, when they had the chance to register their lack of consent, they did not take it.

If consent is crucial and the removal of the organs cannot be justified without it, we should treat consent with appropriate seriousness. Only the person concerned can give consent and it can be given only explicitly.

Our relatives and friends cannot consent on our behalf. Their beliefs about what our wishes were are, even if true, irrelevant. Only an opt-in system is acceptable if consent is crucial in this context.

However, it might be argued that consent is not crucial for the justification of using the organs of dead people for transplants. In that case, we need not adopt either an opt-in or an opt-out system.

Whatever view we take, we should be wary of adopting a bogus notion of consent. To dilute the concept of consent is dangerous and inappropriate.