Our justice system is enduring rough treatment right now and not only from some in government.

Reaction to the Zuma affair, and in particular to the decision of the national director of public prosecutions not to prosecute the deputy president, seems to make some questionable assumptions about our politics which may also cause us to misunderstand what is needed to build a working justice system.

First, much discussion of the Zuma saga seems to assume that we already have a common understanding in our society of what justice is.

We therefore hear repeatedly that Jacob Zuma's political career is in grave jeopardy because of the accusations against him, but this is not automatic. In fact, it may be plain wrong.

In divided societies, where citizens do not share a common sense of justice and a trust in the system that implements it, a political figure accused of wrongdoing is not certain to lose support.

In the US, accused politicians have sometimes gained it. White governors in southern states and, later, at least one black mayor, were re-elected after being accused or convicted of crimes. Both white southerners and black people in US inner cities believed, for differing reasons, that their representative was being victimised by an establishment that meant them harm. To them the charges or conviction showed the reigning powers were trying to get at their leader, which showed the representative was doing a good job. Clearly there was then no shared belief in the US that the justice system was applying fair rules fairly.

We too lack a shared sense of justice, so it is hardly a foregone conclusion that the saga will hurt Zuma if he is not convicted or found by Parliament to have acted improperly.

That understanding is important, and not only to accurate political analysis. It also points to one of our society's challenges: we are not defending a justice system whose credibility is undisputed, we are trying to build one that will win widespread respect as well as a sense of justice the vast majority of us can support.

How do we build it? One way is not to jump to negative conclusions about the way the system is working.

It has been assumed widely that Zuma has not been prosecuted because the governing party pressured the authorities not to charge him.

In the absence of evidence, this conclusion may also be based on a misconception the apparent view that the African National Congress (ANC) is a monolith that will close ranks behind anyone who is accused.

In reality, the ANC is often divided. Since 1994 scandal has usually reached the media because someone in government wishes to weaken someone else. Some in the ANC are already beginning to look to the election of a new president in a context in which there is no clear successor, suggesting that attempts to discredit opponents are becoming more likely.

In one sense, we should be grateful for that because without it we would probably never learn of wrongdoing in government. But it does give cause for pause about theories that assume a united ANC attempt to stop a prosecution. It seems highly unlikely that everyone in the ANC would prefer Zuma not to be prosecuted. This raises questions, in the absence of clear evidence, about suggestions that the governing party united to protect him.

It is also, of course, why his supporters may continue to back him they might conclude that he is being victimised by powerful opponents.

This political reality means that there may well at times be smoke without fire. If corruption allegations are tools in political battles, it is hardly out of the question that some might be false. The assumption that all allegations must be true and that any failure to prosecute shows political pressure on the justice system may be based on flawed political analysis.

This does not necessarily mean that the decision not to prosecute Zuma was correct public debate on this, and the search for new evidence, will and should continue. But it does mean that if a mistake was made it was not necessarily the result of political pressure. And we do our justice system no favours when we impute motives to it based not on evidence but on dubious political surmise.

Unless we want a system in which everyone accused of anything is assumed to be guilty we must allow for the possibility that prosecutions may be withdrawn because prosecutors feel, rightly or wrongly, that there really is not enough evidence.

Assuming political motives for prosecutors' decisions without evidence may undermine public credibility in the criminal justice system for no good reason. It may also draw attention away from a real threat to that system.

While there is legitimate doubt about whether and why Zuma should be prosecuted, the same cannot be said of Tony Yengeni, Winnie Madikizela-Mandela or Allan Boesak.

All three have been convicted. And yet it has been suggested that the first two should not go to jail while Boesak continues to attract attention to himself by demanding a pardon.

In the first two cases, their political roles are cited as reason for leniency, creating the suggestion that convicted "struggle" figures should not be treated in the same way as other citizens.

If any of them is treated leniently, the justice system's credibility will indeed be under threat.

All three were convicted in open court, and all had adequate legal representation something that many people cannot afford.

To treat them differently to anyone else convicted of a crime would send a message that some of us are indeed more equal than others and set back the attempt to build widespread support for the criminal justice system.

Building a shared sense of justice is a difficult task that faces many obstacles. It is not helped by imputing motives to public figures without hard evidence or by suggesting special treatment for people who have been convicted by evidence led against them in court.

Friedman is senior research fellow at the Centre for Policy Studies.

With acknowledgements to Steven Friedman and the Business Day.