In chapter VI of Political Liberalism – the Idea of Public Reason – Rawls continues to build off of the previous posits of the fact of a plurality of reasonable comprehensive doctrines and the possibility of an overlapping consensus. In order to answer a potential paradox, right at the outset, of positing a public reason, he appeals to these two previous posits. That is, the plurality of reasonable comprehensive doctrines and the possibility of an overlapping consensus coming together to ground the liberal legitimacy: citizens affirm the public reason from within their own reasonable comprehensive doctrine.
Well and good. Here’s a thought, however, that one might have at this point. As Rawls points out many times, the public reason sets sharp limits on what is affirmable, or acceptable, within the political society (especially in regards to the constitution of a democratic society). Now, these limits might irk some and cause others to dismay, but Rawls assures us (see chapter V) that given enough time public reason gains moral legitimacy and moves beyond being a mere modus vivendi. Everyone’s reasonable comprehensive doctrine will alter so as to adjust to the demands of the public reason and what a liberal society requires. That is, the reasonable comprehensive doctrines will become more reasonable and, presumably, more comprehensive, once everyone internalizes the public reason.
What seems prima facie odd about this is that it is, prima facie, backwards. One might (naively?) think that it should be the public reason, not the reasonable comprehensive doctrines, that must alter over time. It is a rather weak and un-comprehensive reasonable comprehensive doctrine that gives ground easily and over time. The very point of a genuinely comprehensive doctrine, I assume, is that it has the answers to political questions and procedure. They will also, each, have an understanding of what justice is. So why should they alter and adjust for the sake of an external ideal – one that is other than their own reasonable comprehensive doctrine – rather than the other way around? I assume this is the difference between a liberal and a conservative political conception.
Rawls has a ready answer: each citizen individually affirms the public reason based off of his or her own reasonable comprehensive doctrine. So, the motivation is already present. Of course, in an ideal theory one can posit almost anything one wishes (I might add myself that in an ideal world every citizen realizes, based on their own reasonable comprehensive doctrine, that they should have a classical education and learn Latin, but I doubt that will make it into the public reason). Fine, but what if they don’t? What if there are reasonable comprehensive doctrines that cannot, for internal reasons, ascribe to the public reason? Perhaps because they see precisely what Rawls wants: they will have to adjust over time to fit in with the liberal society. Are they thereby unreasonable? If they are, by that fact, then I hope it is obvious that the liberal legitimacy is not exactly legitimate: it begs the question that all reasonable people will accept the public reason, even based on their own individual reasonable comprehensive doctrine.
Is it not at least conceptually possible that there can be a reasonable comprehensive doctrine that does not, for reasons internal to it, accept the public reason? Of course it is – but then something has got to give. That this is the case I shall develop and defend for the remainder of this brief essay. The key is to go back to the paradox on page 216 of Political Liberalism. One might ask of Rawls, essentially, why it is the case that in fundamental matters in society one is limited to the scope of public reason – for decisions, discourse, voting, etc. Why not, rather, use all of what one takes to be the truth of the matter to make a decision? Indeed, it seems rather foolish not to do so. Rawls’ answer, as seen above, is that such is simply to impose one’s private values onto the public – and that is coercive. Public values, in fundamental matters, must always trump private (or even not-so-private?) comprehensive doctrines.
As an example of this significant and all-telling point Rawls gives one example (to my knowledge, the only example in the book). This is in footnote 32, on the “troubled question of abortion.” Any reasonable balance of public reason will hold that abortion in the first trimester of pregnancy (and, probably, much later on) is permissible and a right of the woman in question. As he states, “The reason for this is that at this early stage of pregnancy the political value of the equality of women is overriding, and this right is required to give it substance and force.” Rawls’ point here is simply that any comprehensive doctrine that excludes this right is thereby and to that extent unreasonable. That is, he directly denies the conceptual possibility raised above that there might be reasonable comprehensive doctrines which, for internal reasons, cannot accept the dictates of public reason. As he puts it later on, “The only comprehensive doctrines that do not accord with public reason on a given question are those that cannot support a reasonable balance on the political issues it raises. Certain reasonable comprehensive views fail to do this in some cases but we must hope that none that endure over time in a well-ordered society is likely to fail in all or even in many cases.” The trouble, however is that this rules out too much. Rawls cannot even allow voting on the issue of abortion from what one takes to be the dictates of one’s comprehensive doctrine, where that doctrine denies the right to abortion.
That it does rule out too much is easily demonstrable, given a plausible example of a reasonable comprehensive doctrine that rules out signing onto public reason for reasons internal to it. It is well known that the Roman Catholic Church, an institution with well over one billion members world-wide, is opposed to abortion. It is likewise opposed to any redefinition of marriage to anything other than the conjugal union of one man with one woman. This latter is a further example akin to abortion, where public reason on Rawls account would hold there to be a right to marriage in various forms. Controversy over the permissibility of euthanasia would also fit in here. The positions of Roman Catholics (and myriads like them, including any philosopher, lawyer, or political theorist who holds to natural law – and there are myriads of these too – and some forms of Kantianism) are held for reasons internal to their comprehensive doctrine. Hence, any good Roman Catholic must, in pain of violating their faith, vote and hold that abortion is both immoral and against the law – there being no right to abortion. Likewise for any change to the definition of marriage, legally and morally. But all of these comprehensive doctrines are therefore, and to that extent, unreasonable and unjust – indeed, Rawls calls them cruel and oppressive. For they deny a right to abortion simpliciter and in all cases, even in cases of rape and incest. These doctrines must be unreasonable and thoroughly unjust, according to Rawls. They should, therefore, be excluded from all political and social practices.
Now, in the quote above Rawls held out the hope that these comprehensive doctrines – which are not going anywhere anytime soon – would be reasonable in other respects. But, how much is needed? I have three hotly debated examples above, and these could be multiplied. These comprehensive doctrines simply cannot, on pain of contradiction, sign up to a public reason of the sort Rawls envisions. Nor is it realistic to hope, as Rawls does, for “leeway” where they can accept the conclusions of public reason – even reluctantly – for it is internal to them essentially that they cannot accept, except on pain of contradiction, the sorts of conclusions Rawls envisions. Rawls wants these comprehensive doctrines to “bend” under a reasonable and effective political conception; but for Catholicism to bend is for it to be destroyed. For natural law to bend under political pressure is for it to cease to be a metaphysical and moral theory – it is for it to become some form of consequentialism. Not even Kantianism can bend and remain Kantianism, this is simply too much to ask! In which case, I repeat that not only is it conceivable, and possible, for there to be reasonable comprehensive doctrines that cannot accept public reason or the dictates of public reason for reasons internal to them, but there are many. Either Rawls must say, ad reductio ad absurdum, that Catholicism and natural law are unreasonable comprehensive doctrines and that they must be excommunicated from a well-ordered society and the realm of political involvement, or his conception of public reason must give way.
 See Political Liberalism, 243.
 I take it that Rawls does not mean merely that it is a legal right, a political right, but also a moral right, given that the political values are supposed to be ratified and held ultimately as moral values of every citizen. This alone should be a reductio against his positions regarding comprehensive doctrines, as legal and political are not merely conceptually distinct but logically and also ontologically distinct orders of reality.
 Ibid, 243n32.
 Ibid, 253.
 Ibid, 244n32.
 Such as myself, and I am not even Roman Catholic.
 Ibid, 246.