Practitioners of Scots law are fond of boasting of the superiority of our system.

They can grow quite philosophical about it. The first thing a law student is liable to hear will have something to do with the balance of principle and precedent, the sophistication found wanting – but let's name no names – in the legal codes of certain other countries.

It's all true, as far as it goes. Scots law is nominated as one of the cornerstones of national identity for good reasons. Despite all the pressures of Union, the old Westminster habit of treating Scotland as a legislative afterthought, and the unspoken demand for assimilation, a tradition endures. Its biggest enemies, in no particular order, are sensational headlines, TV courtroom dramas and reforming politicians.

Take not proven, Walter Scott's "bastard verdict". It allows the idea that in a criminal trial there might be something between guilty and not guilty. The prosecution's job is to prove a case. If the effort fails, and innocence has not been demonstrated, the Scottish verdict makes perfect sense. But not proven is despised. To many it's a pathetic technicality, one that – here come the headlines – let's "the guilty" walk free.

Sometimes it does. That is a failure of the prosecution, not the verdict. A shameless rapist might remain at liberty because the evidence and arguments failed to make the case. The crime was beyond proof. So what would we prefer? Someone locked up because he "obviously did it"? Too often, that has become the cry.

You can understand the impulse. The victims of sexual violence are betrayed on a near daily basis. Often enough they are abused twice over, first by the attacker, then by the machinery of justice. But if you decide to alter the meaning of "reasonable doubt" just to serve an understanding of what we believe we know, or think we know, or believe must be the case, a big risk is run.

Corroboration is another of the old quirks of Scots law. Some hills are not as old as this one. Most of us respond, meanwhile, to a biblical idea almost by instinct. In essence, no-one should get done thanks to the testimony of a single witness. That's why, in Scotland, we have two cops patrolling together rather than one. "Your word against his" offends that old Scottish fancy for natural justice.

Corroboration as a principle does not always serve the truth, however. How many witnesses are there liable to be in a case of rape? In these crimes, as often as not, the attacker's entire purpose depends on escaping observation. "Your word against his" stands, time after time, at the heart of a defence liable to treat the victim as a person to be destroyed rather than heard.

Scotland's record of rape convictions is pitifully "low". The crime is vastly more prevalent than the number of men put inside for uniquely vicious behaviour. Were convictions made easier, some of those with an ugly need to do violence to women might be deterred. That's not a good enough reason to throw aside an idea of proof that exists in no other legal jurisdiction.

There's an irony, no doubt, in the fact that an SNP administration is hell-bent on getting rid of the idea of corroboration. Kenny MacAskill, Justice Secretary, has taken the recommendations of Lord Justice Clerk Carloway and run with them. Another piece of distinctive Scottish jurisprudence is being chipped away because "reform" looks like an easy answer to hard cases. There has to be a better way.

You could applaud the Minister for supporting the jury system – another principle assailed below the Border – with the suggestion that the definition of a majority verdict should alter from eight jurors of 15 to nine or 10. That too could have unintended consequences, but the problem will keep. Do we seriously propose to secure convictions on the basis of one person's word? If so, claims that justice has miscarried will clog up the courts for decades.

That remark is not meant to split hairs. How would a professional defence team, bound to seek the best outcome for a client, behave if corroboration was abolished? An argument over the word of A against the word of B will get you into the European legal arena time after time. Victims will be attacked with a new viciousness – this is probably saying something – as a matter of course.

These are things a defence must do. Those who defend the rights of rape victims are disgusted by the fact. Given some of the cases that have passed through Scotland's courts, you can hardly blame them. The 2010 Supreme Court ruling involving Peter Cadder has meanwhile become a real challenge for Scots law. But you cannot cast aside the idea, offensive as some claim to find it, that the accused has rights.

Remove the argument over corroboration, for a second, from the debate over sexual violence. Where might reform leave the rules of evidence in other types of cases? The word of one witness – a police officer, say – against a well-known wrong-doer? Let's call the latter me.

The circumstances by which I could get lifted, charged and convicted are expanded vastly if it is his word against mine. I would still have "beyond reasonable doubt" and an increased number of jurors to count on, but the removal of corroboration from the defence's deck of cards would not exactly enhance natural justice.

Ministers have no intention of allowing police practice to become nefarious, of course. They have a humane end in view. Mr MacAskill, as a stout Nationalist, has no desire to degrade the traditions of Scots law. But the Minister is caught regardless in an old paradox. He means to advance some liberties by diminishing others. That isn't progress.

At a conference in March, Sandy Brindley, national co-ordinator of Rape Crisis Scotland, said: "Rape Crisis Scotland believes the current requirement for corroboration presents an unacceptable barrier to justice for rape survivors. Most rapes are carried out by someone known, there are rarely any witnesses and significant physical injury is uncommon. Only around 25% of reported rapes get to court ... We are also concerned it means there is a real possibility of guilty men walking free."

So would the abolition of the corroboration rule improve the conviction rate? There is no evidence, from other jurisdictions, of that. Corroboration is meanwhile a feature of Scots law so distinctive it counts as unique. No-one else adheres to this old principle. Does that leave us out of step because rape convictions are hard to secure, or better placed than most? Scotland's Government seems hell-bent on this reform, but the precedent about to be set is hellish.

Any effort to improve the chances of a prosecution by hobbling the defence is bad law. In some societies, it becomes nastier still. Corroboration matters as a fact of logic and of life. At the heart of any justice system there is a simple phrase: "Prove it". The reforms proposed are not restricted to crimes of sexual violence. Why is that, exactly?

Two witnesses to say a crime was committed: all difficulties aside, is anything less than that good enough?