It isn't often that a phrase from a dead language captures the interest of the nation. This is what the Latin words "prima facie" have done, especially after they were used by Bulelani Ngcuka, the national director of public prosecutions.

He was delivering the decision he had reached in the matter of Deputy President Jacob Zuma and the arms deal debacle. He told the nation: "We (meaning the Scorpions investigating unit) have concluded that while there is a prima facie case of corruption against the deputy president, our prospects of success are not strong enough.

"That means we are not sure if we have a winnable case. Accordingly, we have decided not to prosecute the deputy president."

What does prima facie mean? And does Ngcuka actually have the right not to prosecute once he announces he has a prima facie case against an individual?

The affair raises all manner of critical issues, not least Ngcuka's behaviour and the wisdom of positioning an investigating unit under a prosecutorial service.

Prima facie means "as it first appears, at first sight, on the face of it, so far as can be judged by the first disclosure". But it actually has much greater import. It also means "such evidence as is sufficient to establish a given fact and as will remain sufficient if uncontradicted".

It is the right to contradict, to rebut, that lies at the heart of this. In the South African dictionary of legal phrases "prima facie proof, in the absence of rebuttal, therefore, means clear proof leaving no doubt".

What Ngcuka has done, therefore, is to insinuate that Zuma is guilty of an offence (corruption) for which he will not be tried. The excuse is that he does not consider he has a "winnable" case.

But actually, and as experienced prosecutors will testify, the "winnable" element is an essential part of the prima facie equation.

If the evidence on which the prosecution intends to rely won't stand up to the prosecutor's own examination, let alone that of the court, then how can anyone possibly claim that a prima facie case exists?

In a discussion on public prosecutors, Lansdown and Hoal (1957) set it out quite clearly. "Where a sufficient prima facie case is disclosed to him, it is the duty of the person charged with the prosecution of crime to take the necessary steps for the prosecution of the alleged offence, howsoever the knowledge of the alleged perpetration may have reached him."

There is no "yes, but " in this. If a prima facie case exists, prosecutors must do their job they must prosecute. The court will decide whether they had a "winnable" case.

Ngcuka has vilified an individual, and has then denied him the opportunity to rebut the allegation. He has also, by the way, denied the state the right to determine whether that individual is guilty or innocent.

In a sense, Zuma lives a double life, both lives intimately wound up with each other.

On one hand he is a career politician who has, until recently, made a pretty good fist of it. On the other, he lives permanently in the court of public opinion from which his rating must be as close as doesn't matter to zero.

The reality is that Ngcuka's decision not to prosecute and the language he used when publicly explaining why have in fact imposed a much harsher sentence on Zuma than any court might exact.

If he wants to remain a public figure, Zuma really has no alternative other than to fight back.

This is why, in response to the charges brought against Schabir Shaik and in which Zuma is clearly an accomplice, he has asked the court to oblige Ngcuka to disclose what must presumably be the lynch pin of the prosecution's case. This has now become swamped in argument about what and what may not be released for public perusal.

Zuma might also like to think about a defamation action if he believes sufficiently strongly about the harm done to him. That is one side of this. The other, about which I have written in another column, is the whole basis on which the existence of the so-called Scorpions is predicated.

It was meant, when it was established, to be a clone of the US FBI and the rather more famous "Untouchables".

The unit has, however, been compromised from the very beginning by its positioning as an arm of the state's prosecuting authority. This country uses an adversarial court system as opposed to the inquisitorial method applied by many European countries.

In both systems complex rules have been developed for the conduct of criminal prosecutions and why it is so clearly wrong for the national director of public prosecutions to lead a powerful investigatory arm.

We need to get back to the three-tier system that separates investigation, prosecution and judgment, as quickly as possible.

This does not mean the Scorpions must be disbanded just that they must kept away from prosecutors who have political ties and obligations and who may be tempted to abuse their considerable powers.

Besides which, it is patently wrong for Ngcuka to head the Scorpions while his wife is a senior cabinet minister. In fact, he should not be involved in prosecutions at all. It is much too incestuous. As to the motives regarding Ngcuka's curious behaviour over Zuma, what is being whispered in African National Congress circles is that it may have less to do with the law than with something else. What, and according to whose agenda?

Ngcuka may protest that his sole concern is about upholding the justice system but his inexplicable actions have had the effect of traducing it. He has behaved as if he has the right to be prosecutor and judge and jury. Then he runs away. Why? Has he lied to us or did he not have a case in the first place?

Meanwhile, City Press is full of accusations that Ngcuka's own past is dark to the point at which it is alleged he was an apartheid era Special Branch spy (agent no: RS 452). Another, not yet given public voice, is that, when he was Durban activist lawyer Griffiths Mxenge's clerk, Ngcuka was a conduit for sensitive information to the security forces.

Mxenge and his wife were later murdered in particularly brutal circumstances.

Whether any of this is true is untested. What is clear is that a terrible row has erupted in the ruling party, one that may tear it apart. It is also the case that both Zuma and Ngcuka are responsible to Parliament for their actions. It is unlikely the courts will resolve this mess swiftly and speed is now of the essence.

Zuma and Ngcuka cannot both continue to hold their current public posts. So let Parliament decide. Ngcuka says he has evidence to prove Zuma's malfeasance and corruption. But he won't take it to the courts.

So, let Parliament, the ultimate legal authority and to which he is responsible, call him to its bar, to enable it to assess the extent to which he is competent to uphold the authority of the judicial system.

With acknowledgement to Business Day.