Education, no less than politics, is the art of the possible. We do not give children challenges that they cannot possibly meet.
Four-year-olds are not expected to study silently for hours at a stretch, and children are not expected to trek back and forth for their schooling between London and Newcastle. Education is about matching what is possible to what is real.
A debate is going on in the UK today which has precisely these themes. It is called the inclusion debate, and it has been simmering for a long time, but it was brought to the boil again as a national issue by Mary Warnock's call for a re-think of special needs policy last summer.
Parties to the debate are divided as to whether all children can be successfully educated in mainstream schools. Full inclusionists say it is possible for every child to get on in a mainstream school. Of course the school must have a fully inclusive ethos - it must be physically equipped with ramps and aids, and it must have a full quota of teaching assistants for those who need them.
Bullying, in such a school, must be zero-tolerated and stigma must be a thing of the past.
Hang on, say the sceptics, notably Baroness Warnock. Some children cannot flourish in mainstream schools, however enlightened their ethos. These schools are too big, too noisy, too bright, too fast-moving. Autistic children, for example, need a sensory environment with noise levels akin to those of a Trappist monastery.
Author Charlotte Moore should know. She has two autistic children and believes that the policy of full inclusion is wrong: "wrong as in misguided, and in some cases wrong as in immoral" (George and Sam, page 170).
You may have to read her book to understand this. She shows that George and Sam need something that the mainstream cannot provide to be calm enough to learn anything, and not to unlearn what they have already learned.
The full inclusion policy is based on a dream. In this dream, all children mingle happily, embracing each other's differences. They are being primed to take these enlightened attitudes into the post-school world where they will create a humane and just society.
There is nothing wrong with this dream so long as we ask if it is also possible. Can all children mingle happily, or are we asking some, like the autistic ones, to travel daily between London and Newcastle?
What is striking is that the people who live and work with autistic kids are painfully conscious of the price of full inclusion. Parents fight the closure of the special schools which they see as a lifeline for their children.
The full inclusionists see things differently, of course, but these are generally politicians and academics who have slight acquaintance of autism.
We have come a long way since the Warnock report of 1978, which ensured that every child had a legal right to an education, in the mainstream wherever possible. Since then, many children who might have been excluded have been successfully included in such schools.
I do not know anyone who prefers pre-Warnock-style segregated education.
Perhaps the dream of full inclusion is partly motivated by a fear of slipping back to those days, but fear is no basis for a national policy.
It is no excuse for denying parents and children a choice, or refusing to listen when they describe their horrific experiences in mainstream schools.
We should not dream the dream of full inclusion. It is optimistic to the point of Utopianism, and it gambles with the lives of children.
If education is the art of the possible - and why should it not be? - we must temper our dreams with a simple injunction: to try to understand the genuine needs of children. This is not always easy, and we should not pretend that it is. In the simple street language that everyone understands, it is time for the full inclusionists to get real.
Dr Ruth Cigman is the editor of Included or excluded? The challenge of the mainstream for some SEN children, with a foreword by Mary Warnock, due to be published by Routledge in November