Previously on the Deep End, we’ve featured Michael Oakeshott’s argument that conservatism is not an ideology – a case also made by the American conservative philosopher Russell Kirk.

Rejecting ideology does not mean rejecting principle – because the two are distinct. While principles are about how the world ought to work, ideologies are about how the world must be made to work.

The trouble for ideologues is that the world often has other ideas. This is something that Ed Balls now knows to his cost. Having drunk the neo-Keynesian Kool Aid, the recovery of austerity Britain left him dead in the water.

Economic policy is dangerous ground for ideologues – hard facts lurk behind every corner ready to jump out and do serious damage to vulnerable theories. It’s different in other policy areas, where outcomes aren’t quite so quantifiable. For instance, in the field of education, ideologies have gone unchallenged for decades, vulnerable only to the rise of rival theories.

Fortunately, things are changing, with arguments increasingly settled by evidence. The growing influence of international comparative studies like PISA is a particularly welcome development. While ideologues may succeed in imposing uniformity within one country, they cannot stop comparisons being made between different countries.

Sam Freeman, a former advisor to Michael Gove, provides an indispensable guide to the latest PISA report. Among his many fascinating observations is this one:

“It’s very hard to not just cherry-pick examples that support one’s existing views. There do seem to be, though, some strong themes around the most successful and most improved countries. One is selection – Germany and Poland are both reducing selection in their systems and have seen improvements and a reduction in the impact of socio-economic status on performance. Singapore is really the only high-performing country to have any selection in their system.”

This is a useful reminder that educational ideology isn’t just confined to the left. For many Conservatives – especially those of us who went to grammar schools – it is difficult to accept that selection isn’t the key to raising educational standards. As much as we’d want it to be otherwise, the evidence isn’t there.

Nick Pearce, director of the Labour-leaning IPPR, also uses his blog to comment on the PISA results. To his credit, he highlights some inconvenient truths for his fellow leftwingers:

“…there is… uncomfortable reading for some of those on the left who advocate a return to schools being controlled and managed by local authorities. PISA shows that ‘schools with more autonomy tend to perform better than schools with less autonomy when they are part of school systems with more accountability arrangements and greater teacher-principal collaboration in school management’. Giving schools freedoms over curricula and assessment can be a good thing – provided it is matched with sound collaboration and management.

“PISA is also clear that simply calling for more money to be spent on education is not sufficient to raise standards. While high-performing systems do pay teachers more, there is a wide difference in which countries get the biggest bang for their buck. Careful thought needs to be given to raising productivity in education.”

This isn’t just “uncomfortable reading” – the facts are doing to Labour’s education policy what the GDP figures did to their economic policy.

Ironically, it is Michael Gove who is condemned as the ideologue. He is in fact the most anti-ideological of Education Secretaries. In rejecting the old rightwing obsession with grammar schools, while smashing through the entrenched leftwing opposition to free schools and tougher standards, it is evidence not theory that guides him.