BBC - Radio 4 Woman's Hour -Male Primogeniture Jenni discusses the law of primogeniture with Liberal Democrat MP Lynne Featherstone, who has asked the Equality and Human Rights Commission to investigate the legality of this centuries old practice, and to royal commentator and author Charles Mosley.

BBC - Radio 4 Woman's Hour -Primogeniture: Should the law be changed? Thirty years since the passage of the Sex Discrimination Act, the law that governs Succession to the Crown in Britain is also based on primogeniture: a male-preference law that dictates the throne should go to a male heir over a female. Why does the law continues to exist and is there a case for changing the law sooner than later?

The really interesting (for this analysis, not for itself) argument in this discussion of male primogeniture was made by Charles Mosley. Several times he presented two 'logical' objections (invoking the term logic) 1) "if you're interested in equality why not prefer the abolishment of monarchy which is even a more startling symbol of inequality?" 2) "if you [Lynne Featherstone] are also a spokesperson for the rights of the young, why not also include a change in the inheritance going to the firstborne?" [my paraphrases] His interlocutor responded by saying "ah, that's the old chestnut that is often used to stop change".

That fact that they were both justified in their remarks tells us something startling about public debate: logic and rationality cannot be and are not what decides the argument. At least, not in the traditional sense. The traditional view of logic is that it is independent of the matter at hand as well as of the arbiter of correctness and can be therefore used as yardstick of the validity of the argument. That's why Mr Mosley could sound a bit condescending (even though that could have been by dint of his being the 'royal' commentator) when he said something like 'then it surely follows' and 'isn't it logical'. Mr Featherstones justified objection that this was not a 'legitimate' argument did not have nearly as much weight purely because of the high regard we have for logic in these contexts.

Yet, it is 'obvious' that in this case the logic is gerrymandered and used as a rhetorical device. But is that always so? There are many times when the 'logic' applies.

Basically, the 'logic' of any argument needs to be negotiated. And any negotiation will bring with it all the factors that 'real' negotiations do. I.e. aspects of power, prestige, location, primacy and recency, ingroup/outgroup concerns, etc. In other words, it is not easy. But it is the only way it can go. This is not a suggestion as to how things should be, merely a description of how things are. There is an open question whether such debates would be more fruitful if both interlocutors were aware that they are engaged in a negotiation and subject to various influences. I'm a bit skeptical on this count but not convinced yet either way.

Let's have a look at other similar examples.

The right wing's argument that if you allow gay marriage what's to stop you from allowing zoophilic unions? Of course, there is nothing that would do that. A perfectly valid modus ponens argument can be constructed to demonstrate this entailment. Yet, intuitively, we know that such a occurrence is very unlikely.

Similarly, an argument can be (and has been) made, that if we stop abortions and outlaw prophylaxis we should also force any young heterosexual couple in an encounter to procreate. For, are they not depriving a potential human life of coming into being by avoiding copulation? Again, logic fails completely. Only negotiation of categories, concepts, and their mappings can bring any results.

Of course, it also works the other way. An fairly undisputably legitimate logic can be overriden by negotiation. The claim against the legalization of marihuana is that it is a gateway drug. Yet, the numbers of people who have smoked marihuana (almost everybody - except, for some reason, me) are so great that if there was any underlying causality significant proportion of the population would be addicted to hard drugs. Yet, that is clearly not the case. Other factors prevailed in the negotiation.

Another example is the British argument against immigration for the reason that 'Britain is a small island'. First, it is neither small nor comparatively densely populated but most significantly, its being an island doesn't make its borders any more solid than, say Germany. If, Belgium needs more space to house migrants, surely, they can't borrow a spare bit of France. Nevertheless, this is a very common argument.

And how about opponents of the smoking ban. It is quite true that smoking does not 'cause' cancer in the way that a bat causes a person to have their skull smashed in. Nevertheless, it was possible to generate a consensus of causality that overrode the logic. (I've heard a similar argument made against evolution.)

And so on, and so on.

But wait! There's more. Not only is not logic a good tool for independently judging the validity of an argument (although it is a good tool for putting an argument forth), it is not a good predictor of cognitive and social outcomes. And this applies to metaphoric implicature, as well. This is due, again, to the fact that these outcomes are negotiated. This doesn't bode well for most social commentary and a good chunk of social science.  But that can be explored some other time.