But their frankness does help us to understand the shortcomings of some of the policies they administer. Back in November, asked to distinguish between ability - the criterion by which grammar-school places are allocated - and aptitude - the test of suitability for specialist education - Lady Blackstone gave a classic answer.
"Researchers defined ability as an all-embracing factor . . . and aptitude as being narrower, determining what someone has the capacity to be trained to develop." Then, in case the magisterial disassociation had escaped her audience, she added: "That may not satisfy, but it is what the researchers came up with."
A couple of weeks earlier, Estelle Morris - replying to a letter from Margaret Tulloch, the admirable secretary of the Campaign for State Education - admitted that, although the Government has "encouraged debate on specialist schools, at present there is no specialised research in this area".
I hope that I detected a note of intellectual regret in Ms Morris's answer. If I was right, she will be delighted to know that Tony Edwards, emeritus professor of education at the University of Newcastle upon Tyne, has filled part of the gap. Stephen Byers, the minister who argued most strongly for the extension of the Conservative bright idea, may not be so pleased.
The purpose of his work, Professor Edwards insisted, was "not to argue for or against the specialist schools' policy, but to review evidence relevant to assessing its more likely effects".
He therefore judged the extension of specialist schools against three highly-publicised claims - parents' demand for greater choice, a clear distinction between aptitude and ability, and the likelihood that creating specialist schools will improve standards generally. None of his conclusions could be described as vindicating the Government's enthusiasm.
Professor Edwards writes so well that there is no better way of reporting his findings than quoting from his reports. "Research into parents' reasons for choosing one secondary school over another has not yet revealed any substantial demand for departures from a traditional academic curriculum, successfully taught."
Why then is the Government offering pound;100,000 to each specialist school - assuming that it is matched by pound;100,000 of private money - and paying an annual pound;100 grant to every specialist school student for the lifetime of the three-year development plan? The only possible answer for a Government which is genuinely opposed to segregated secondary education is that specialist schools improve the standards around them by what, in the jargon, is called "curriculum diversity".
On that subject, Professor Edwards is academically ambivalent. Judgment is complicated by the likelihood that they will attract "pupils from whom academic success is easier to obtain". That sounds depressingly like the allegation that specialist schools are - whether the Government admits it or not - selective.
Professor Edwards's conclusion confirms that gloomy suspicion. "Specialisation is hard to separate from selection, certainly in conditions where schools compete for pupils . . . Selection by interest also tends to produce socially-segregated intakes."
Of course it does. Every initiative which is required to obtain a special sort of education - application, examination or interview - creates a barrier across the path of the most disadvantaged children.
The hard fact is that specialist schools will add to a hierarchy which already guarantees that the articulate and self-confident middle classes can talk their way into what are popularly supposed to be "the best schools". And, if Professor Edwards is right, they will gain that extra advantage without bequeathing any certain benefit to the system as a whole.
Education ministers must know by now that, in Professor Edwards's words: "Low-status parents may be channelled towards neighbourhood schools which have no specialised curriculum offerings and poor resources."
And they still encourage schemes which produce that result. Yet last week, David Blunkett prohibited both the extension of formal secondary selection and (perhaps more important) the pre-entry interviews which some headteachers use to smuggle academically gifted, and socially acceptable, pupils into what are supposed to be comprehensive schools. It is all part of the paradox that runs through New Labour's education policy. No selection by examination or interview? Up to a point, Mr Blunkett. Up to a point.