This is good advice on most subjects. For example, anyone who doubts global warming should consider the plausibility of the sceptics’ claim that what most influences scientists is the availability of funding. This may be true, but it proves the opposite of what the sceptics think. If we are talking money, the amounts available from the oil industry far exceed those available from manufacturers of windmills and other green technologies.

The “follow the money” rule helps us to understand the arguments about inclusion of special needs children in mainstream schools. These have been revived by a Cambridge university report, commissioned by the National Union of Teachers. It concluded that, for many teachers in mainstream classrooms, their jobs had become “more like nursing than education”.

The story goes back to 1978 when an official committee under Mary (now Baroness) Warnock expressed what was then the progressive wisdom, which I shared. Physical handicaps, complicated medical conditions, mental illnesses, uncontrolled bladders, and a propensity to plunge knives into your classmates’ groins - all these should be regarded as part of the rich tapestry of life.

The majority of children would benefit from living and working alongside these problems in their formative years. It would create a kinder, more tolerant society. In any case - and this was Warnock’s most important point - many more children than those in special schools suffered disabilities that affected their learning. It was unfair to label one group as “handicapped” or “maladjusted” and teach them separately as though they were some alien species. It was equally unfair to treat all other children as though they had no special needs whatever.

But look at the date of the report: 1978, when the post-1945 social democratic consensus was coming to an end, and Thatcherism was about to take the stage. Now follow the money. The belief that the first duty of governments was to keep down taxation and restrict public spending became dominant for the next quarter-century.

Special schools are expensive. The liberal belief in inclusion could live in happy, if accidental, harmony with the conservative belief in low spending. The political correctness of both left and right conspired to end separate schooling. Something similar happened in mental health. What used to be called lunatic asylums - Victorian institutions that would have cost a fortune to replace - were closed in favour of “care in the community”.

It is all very well for proponents of inclusion - whether of special needs children or mentally ill adults - to say they had envisaged more, not less, spending. The reality was that governments would never spend enough to make inclusion a success.

I still believe inclusion was right in principle. But since 1978, it is not just lack of money that has prevented its success. The whole culture of schooling has turned against it. Children must be driven relentlessly from test to test, and nothing - certainly not a classmate’s need for a tracheotomy - must stop them achieving the highest possible scores.

Parents enjoy more rights to choose schools. Many will avoid those that have significant numbers with certain types of special need. The idea of inclusion belonged to a more socially conscious, less meritocratic era, where education was not seen purely as a vehicle for competitive advantage.

We have created a school system that makes inclusion impossible.