As every teacher knows, you can understand something in theory, or you can learn it through hands-on experience - and no prizes for guessing which is the more powerful lesson of the two.
Like most people who've thought twice about British education, I've always known that our two-nation school culture - we have the largest private-school sector in the world - must sap the national will to build a strong state education system. Logic dictates it. But it is only when you become a parent with a foot in each nation, and a direct view down at the fault line below, that you come to realise just how true that is.
Is - and will be. Because according to the latest figures from the Independent Schools Information Service, private-school pupil numbers are up for the first time in five years, and growing fastest at the prep school and nursery levels - levels unclouded by incoming overseas students and therefore directly reflecting the choices being made by British parents as the recession recedes.
Our public-school system is here to stay. As in previous generations, about one in 14 of today's schoolchildren - a hefty slice of tomorrow's employers, entrepreneurs, policy-makers and professionals - will grow up in their own private world, learning startlingly little about the life of their peers. In turn, those peers will know little or nothing about the standards and assumptions that shape this influential minority.
Because, as I've discovered after two years with children in both systems, these worlds are not so much two nations as two parallel universes.
If, for example, you move a child from a primary school to a prep school, many more things change than simply the way that child spends her school day. At a stroke, the entire family's timetable, geographical orientation and local network is turned on its head.
Gone are the afternoon walks to the school playground; in come the late-in-the-day drives to a distant campus. Your school holidays are different - you take advantage of the local swimming pool "before the state schools are out" - and your social life is changed. Some primary school parents are offended by what you've done and cut you dead, while dinner invitations start to arrive from parents of your children's new friends. Your child's old friends slowly start to drop away. The friends are busy with town things - tap classes, theatre club - while she is more and more absorbed into the ways of her new school.
It's all very tribal. Us or them. Friend or foe. Like it or not, society seems determined to catch you up and spin you off into another world until, eventually, even your most basic interests, assumptions and expectations start to shift into line with these new realities.
National debates about education - school funding, say, or disruptive pupils - are now of only passing interest. You've become a client, with a client's mentality. You've signed the cheques and what you've bought is the freedom not to have to worry any more about resources, discipline or academic standards. (Although should there ever be a problem, you expect the school to jump smartly to attention, however much it thinks you might be talking through your hat.)
Eventually even your most reprehensible prejudices adapt to fit the new situation: whereas you once felt sorry for those corralled prep- school children - at school from dawn to dusk, no local friends to speak of - now you look askance at the Year 6 troublemakers spitting and swearing their way down the High Street and thank goodness that your child is free of all that.
If you have children in both systems, you can at least bounce between one set of horrible prejudices and the other with some awareness of the gulf that yawns between.
Thus, in a day spent walking the line between the two worlds, you might find yourself standing beside a primary parent as they scoff at the army cadets from the neighbouring independent school. ("Love dressing up and playing soldiers, don't they, these private schools? Catch me paying good money for that. ") Then later listen to a private school father lambaste state school standards. ("I've taken on three trainees in the past six months. Do you know, not one of them could spell 'invoice'? One of them couldn't even spell 'really'. What do they do all day, these schools?") But not many people tread this frontier, or cross from one side to another, and it is depressing to realise just what a wasteland of dislike and indifference divides the two worlds. State school parents often resent private schools and like to think they are about nothing but status and privilege, while to parents born and bred within the private system, state schools can often scarcely exist at all. Some drive past them every day and quite literally don't see them. As for the most exclusive few, their map of the school world is a very sketchy affair indeed, a mere handful of acceptable names.
What happens when children grow up among such schisms and divisions? They perpetuate them, that's what.
Private schools are powerful engines of social control, able - with their high achieving, carefully defined, selective cultures - to mould their pupils far more effectively than state schools can. And if this is the overwhelming influence on your young life - if this is what you feel has made you what you are and given you the things you value - then this is almost certainly what you will choose for your own children. Why, after all, would you do otherwise?
With the consequence, of course, that nothing ever changes. Society continues on its way just as it always has, with the privileged few having no idea how the schools for the many work. And therefore - even though many of these few are powerful decision-makers - unlikely ever to have any real interest in supporting them, or any gut commitment to making them better.
It is usual when writing columns like this to try to end on a positive note, to outline some pointers towards a possible way forward. But I see no end to these separations and exclusions. I fear it is just the British way. How we are. What we do best.
And it is what has made this country what it is and has always been - fine country houses and grim estates; exquisite private gardens and squalid public spaces. And world-class independent schools - and a state education system that these days barely crawls into the middle of most international school league tables.