The idea of an objective order of constitutional value seems connected to the South African doctrine of "objective invalidity." As explained in Minister of Health and Another v New Clicks South Africa (Pty) Ltd and Others (CCT 59/04, 30 September 2005) (the "declaratory relief" decision that accompanies the main New Clicks judgment), this doctrine holds that if a statute is unconstitutional, it has been unconstitutional since the adoption of the new Constitution or, if the statute was enacted after the constitution's adoption, then since the date of enactment.
As a matter of theory, the doctrine of objective invalidity says that the Constitution's meaning never changes. Its interpretation may change, certainly, as judges develop over time their understanding of what the Constitution declares, but these are not the changes in the Constitution's meaning itself -- just in the judges' perception of it.
At some point in a constitution's history, this theory must become untenable. A law enacted in the United States in 1796, for example, may turn out to be unconstitutional when appraised in 2009, but it doesn't really make sense to say that it in fact was unconstitutional from 1796 on -- since two centuries of affairs have been arranged on the basis of its constitutionality. It's too late to unscramble those eggs.
This isn't news to South Aficans, of course. As a practical matter, as the New Clicks judgment makes clear, the "inexorable effect" of this doctrine is readily mitigated in South Africa -- perhaps more readily solved than comparable problems in the US might be -- by the Constitution's grant of discretionary power to courts to limit the retroactive and even to some extent the prospective effect of judgments of invalidity.
But there is another theoretical implication of this doctrine that in the end may be more important. In principle, the doctrine of objective invalidity implies that the meaning of the Constitution -- the actual, true meaning, not just the judges' imperfect perception of it -- cannot change. A law could not truly be constitutional at one time, but later become unconstitutional because the constitution's own meaning changed. (I'm not referring to changes by actual amendment of the text, by the way; presumably with those, the moment of objective invalidity is the date of the adoption of the constitutional amendment. My focus is on changing meanings of an unchanged text.) It must follow, in principle, that a change in the values or circumstances of South African society cannot produce a change in constitutional meaning -- except by formal amendment.
One can imagine such a legal world. Probably Justice Scalia would argue that this description in fact embodies the correct understanding of what a written constitution does, and he'd have John Marshall in Marbury v. Madison, the founding case of US constitutional law, to cite in support. But I think that if the terms of a written constitution are more or less permanent, then over time their meaning simply must and will change, because meanings from decades or centuries ago gradually become inaccessible, inapplicable and likely unpalatable as well. Our framers didn't know this, it seems to me, because they had not had the experience of being part of a nation governed by a more or less permanent text over the long term; they started that process, but we are the ones who've seen its evolution.
One solution would be not to have a more-or-less-permanent constitution; with frequent amendments, textual provisions could be kept up with the times. But this solution I think is worse than the disease, because it would mean that fundamental guarantees are at any moment subject to impulsive rewrite. The genius of a written constitution isn't that its meaning doesn't change, but rather that its meaning doesn't change impulsively. The relative gradualness of the judicial development and revision of constitutional meaning is an important protection against loss of the hard-won protections a constitution is meant to secure -- even though sometimes judicial evolution isn't gradual.
All that said, South Africa may not be at the point where judicial interpretation inevitably, insensibly, begins to take over from original consensus. Not much time has passed since the Constitution was approved in 1996 (or since the adoption of the interim constitution -- the first post-apartheid constitution -- and democratic elections in 1994). But just as South Africa's courts have in only 15 years adjudicated a tremendous range of claims of constitutional right -- essentially covering in 15 years the ground that has occupied the US Supreme Court for two centuries -- so the process of judicial creation of constitutional meaning is likely already under way. Indeed, even in the slower-paced early years of US constitutional history, it's probably fair to say that John Marshall had embarked on this process as well, despite his emphasis in Marbury on the permanence of the meanings inscribed in the written constitution.