Warning: incoherent ramblings of a former mathematician... Feel free to ignore.
Edit:
@Jonathan Edwards post above was being written while I was writing this. Read him in preference to my ramblings! Thank you
@Jonathan Edwards. Admirably clear as ever.
.....
This situation reminds me a bit of studying maths, in particular I'm thinking of geometry. As a child I learned Euclidean geometry, where there are a certain set of axioms accepted as true, for example that parallel lines never meet. Then all the rest of the geometry could be proved using those axioms and deductive reasoning.
Then at University I studied other geometries like projective geometry where one axiom is changed, in this case parallel lines meet at a point at infinity (don't ask). This gave rise to a whole set of new and beautiful new theorems (think art with and without perspective).
Then there's topology, where distance doesn't matter, it's all about whether two points can be joined by an unbroken line along a surface (think Moebius band).
To a topologist, a teacup is the same as a donut because it is a solid shape with one hole.
The point is, completely different views of the world are developed completely logically using logic and deductive reasoning, with just a single axiom (assumption) changed.
In ME/CFS the psychiatrists/psychologists of the BPS school start with the axioms
BPS Axiom 1. that ME/CFS is a creation of the mind, possibly triggered by something physical, but perpetuated by false illness beliefs and incorrect behaviour (inactivity) causing deconditioning.
BPS Axiom 2. that questionnaire data is as real and reliable as biological data.
Everything they deduce in their research stems logically from these 2 axioms/assumptions.
In their world they are being perfectly logical when they conclude, using axiom 1, that a patient who is both fatigued and inactive the causal direction is inactivity causes fatigue.
Using axiom 2. they deduce that questionnaire data is not subjective, is measured on linear scales, fits a normal distribution, or can be treated as if it does, and is reliable, repeatable and not subject to influence, and can be analysed using the same statistical methods as linear biological/physical data.
The biological school starts from the axiom that there is an ongoing biological cause of fatigue and the other symptoms, and the causal direction is from biochemistry to fatigue to inactivity.
Each school of thought has its own internal logic. The problem for us is that the axioms on which the BPS school is built are false in the real world, however 'beautiful' it may be in their imaginary world.
They therefore treat their questionnaire data the same as linear physical data, use statistical tests that are inappropriate, see no need for blinding in trials, and make deductions that fit the internal logic, but are wrong because the axioms are false.
By applying these false axioms to the real world they are doing immeasurable harm, just as if we tried to apply topology to the real world and defined teacups as the same as donuts we might have a problem or two.
End of ramble.
Edit: I should have added the obvious chain of deduction that is so dangerous in BPS (starting point in this logical pathway depends on whether CBT or GET is used):
BPS model logical deductions:
Changed beliefs leads to increased activity leads to increased fitness and decreased fatigue leads to recovery.
Biological model, the logical deductions:
Changed beliefs leads to false confidence leads to increased activity leads to increased biochemical problems leads to increased fatigue and other symptoms (ie PEM) leads to relapse.
Each of these is a perfectly logical chain of reasoning. But one of them starts with a false axiom.