I'm still digesting what happened in the fairies thread. Here is something that's come to mind lately:
Orr's law applies to meditation: What the thinker thinks, the prover proves.
Or, as I'd like to rephrase: What the believer believes, the prover proves.
In this case, the “believer” function can be considered the sum total of one's thoughts and beliefs about meditation while the “prover” function consists of whatever is experienced during meditation.
One obvious example are the brahma viharas: Thinking nice thoughts encourages nice experiences, which reinforce nice thoughts, even off the cushion, which again reinforce nice experiences, even off the cushion, which makes thinking nice thoughts on the cushion easier & more plausible etc.
Another example is insight practice: Believing that reality consist of tiny super-fast arisings and vanishings makes reality appear to consist of tiny super-fast arisings and vanishings, which reinforces the belief, etc. Or in more detail: Accepting the premise that reality might consist of tiny super-fast arisings and vanishings and acting as if that might be true and might be verifiable through observation will lead to the belief that reality consists of tiny super-fast arisings and vanishings and that this is verifiable through observation.
The general formula could be stated as: Thinking something and acting as if it were true changes experience in ways that reinforce the thought, until it becomes a belief that does't need reinforcing.
In pragmatic terms, the question then becomes whether and how thoughts and beliefs can be controled / changed so that we dont believe in things we don't want to be true (i.e. "I'm a failure" etc.).
This applies to meditation practice, but it could be applied to many other things with interesting results. (I have tried some, and all were worth it. What is your experience?) It is also paralleled by the infamous "you get what you optimize for".
As far as I can see, there seems to be no exception to this rule, apart from what Alan Chapman calls the available “means of manifestation”, i.e. some things just don't seem to be possible due to the way this universe is set up. Also, Orr's law can be tricky: Convince yourself that it doesn't hold and see what happens. And: Convince yourself that it doesn't hold FOR YOU, while it does for others, and then see what happens. (This one shouldn't be so hard

)
With regard to meditation, I propose the following experiment, in which two of the endlessly recurring concerns around practice are addressed:
(1) the value of meditation practice (whatever that means to you) and
(2) one's personal ability to practice correctly (i.e. to get the results one is supposed to get)
1. Convince yourself that meditation is very good for you and you are good at it. Go meditate and write a short report. Repeat once a day for one week.
2. Convince yourself that meditation is very good for you and you suck at it. Go meditate and write a short report. Repeat each day for one week.
3. Convince yourself that meditation is useless and you are good at it. Go meditate and write a short report. Repeat once a day for one week.
4. Convince yourself that meditation is useless and you suck at it. Go meditate and write a short report. Repeat.
5. Review & compare the reports of all four weeks, choose the belief set that gives the results you like best and stay with that, discarding everything else.
If you keep a meditation log, you may not need to perform (all parts of) the experiment because you have very likely already gone through & reported about (most of) the iterations of belief listed above.
Have fun

Christian