I've been talking to some friends who have some rather odd spiritual (in the sense of disorganised religion) beliefs. Odd because its a combination of modem philosophy LW would be familiar with (acausal commuication between worlds of Tegmark's level IV multiverse) ancient religion, and general weirdness. I have trouble pointing my finger at exactly what is wrong with this reasoning, although I'm fairly sure there is a flaw, in the same way I'm quite sure I'm not a Boltzmann brain, but it can be hard articulating why. So, if anyone is interested, here is the reasoning:

1) Dualism is wrong, due to major philosophical problems as well as Occam's razor

2) I think therefore I am, so I know that the 'mental world' exists.

3) Therefore Idealism is true, the mental world exists but the physical is just an illusion

4) In response to 'so why can't you fly?' the answer is a lack of mental discipline: after all, its hard to control your thoughts

5) If two different people existed in the same universe, there is no reason why they would perceive the same illusions.

6) Therefore, each universe consists of one conscious observer and their illusory reality

7) But Tegmark's level IV multiverse is true, so we can acausaly communicate between worlds, in fact all conversations are actually acausal communication between worlds.

8) This also implies there is reincarnation, of a sort - there is no body to die, so you just construct a new illusory reality.

From here on it gets into more standard 'spiritual' realms, although I did find it amusing when my friend told me that there are at least aleph-2 gods.

I should state that these beliefs are largely pointless, in that its not obvious that they actually influence any decisions the believers make, and that they do seem to make people happy without any major downsides.

I should also make it clear that I don't believe this, because I wouldn't want to lose status as a rationalist by believing in something unpopular!

TL;DR

To a large extent, this boils down to: how do I distinguish between the hypothesizes that the universe is lawful, and the hypothesis that the universe is determined by my beliefs, and I believe it to be lawful.