Figures out in The Times (£) today show that since the Debby Purdie case, when the DPP was forced to issue prosecution guidelines, 30 cases have gone unprosecuted. On the face of it, this is a cause for celebration.

But there are two aspects to this that set a very bad precedent. The first is summed up best by Lord Falconer, a leading campaigner in the Lords:

But it is right to say that change had already occurred before the publication of the guidelines. What the guidelines did was in effect to codify a change already taking place.

Whilst this shows a move in the right direction, it also shows that laws are being made by a process totally separate to that of Parliament. Assisted suicide remains illegal; until it is legalised in statute it ought to be prosecuted. The law should not be changed incrementally by prosecutor’s guidelines. It would not take much for public opinion to swing violently against something else that could be subtly altered with guidelines. This puts further pressure on parliament to give us the ultimate right to control our lives; and for them to control our laws.

The second aspect that disturbs is much greater. Since Tony Blair came to power old certainties in the criminal law have be chiselled away. Detention without trial is the most well-known, but juries were jeopardised, and a huge number of offences were introduced that could be fined on the spot, with no proof of intent.

And proof of intent intent is the old test in our law – mens rea. For an act to be criminal it must be accompanied by the right intent. In theft as well as the taking of property belonging to another you must prove dishonesty and the intent to permanently deprive. There are tests of recklessness, so as to prevent people from relying on the old “I didn’t mean to” excuse. But it is important that motive is kept out of it. Stealing to feed your self, or to get a new phone is all the same.

But these guidelines focus on motive. Mrs Bateman’s husband was not prosecuted because she had a clear and settled wish to die. But the report goes on to say:

It was also clear that Mr Bateman was wholly motivated by compassion. He cared deeply for his wife and had taken care of her daily needs for several years. There was no evidence to suggest any motive other than compassion.

Many a thief has been motivated by something more decent than mere criminality; and if we are to maintain the vital distinction between assisted suicide and murder then motive ought to be ignored. What matters is whether the intent is to assist a suicide or commit a killing, not why it is done. Serial killers often believe they have strong moral motives.

We must be firm in our resolution. As I wrote recently, we want the freedom to kill ourselves, assisted if needed; but we also don’t want to open the door to the state examining our motives rather than our intentions. Assisted suicide would be a small gain for libertarians; prosecution by motive would be a huge loss.

It is not obligatory to save someone from drowning. It is not a crime to let them drown, as it would be to hold their head under. But it is not a huge leap of the imagination to suppose that if motives were suitable grounds for prosecution then whilst your intent was not to kill someone by not saving them, your motives were bad. Prosecution by motive removes the high barrier of having to prove that someone did and intended to do something wrong.

This is the start of a slippery slope towards making it easier for the state to deprive you of your liberty on spurious grounds. It is why guidelines should not make the law, parliament should.