It seems like (a) and (c) are easily granted, but what's your definition of "non-arbitrary", and how should we determine if that definition is itself a non-arbitrary one?

This topic is one I enjoy thinking about so thank you for your post :)

Another factor is that Christianity is exclusive - one could not adhere to Christianity and, say, Mithraism at the same time, since Christianity claimed a monopoly on religious truth. Other saviour cults which did not function in the same way would not have been able to work up the same amount of religious fervour, since a man's trust in his religion is limited by that religion's trust in itself.

reading about the topic on Wikipedia

Just because there's an article on the spread of Islam doesn't mean that a balanced quantitative analysis on the means of its proliferation either exists or is possible. Usually when someone asserts something to that effect, the onus is on them to support their assertion by referencing a specific source.

This idea seems to be more or less taken for granted by people who oppose either Islam. Is there actually a perspicuous source of data describing in detail how Islam spread, that allows assessments of that kind to be made?

Isn't "aptitude" simply another word for the genetic, non-environmental component of a talent?

Maybe there's a confusion being caused here by the sentence "This is not how evolution spreads."

It could mean at least one of the following: 1) "This is not how the theory of evolution itself was spread" 2) "This is not the mechanism according to which evolution spreads ideas"

It seems as if Lumifer interpreted your statement in the second sense (as I did initially), whereas reading your post in its original contexts suggests the first sense was the one which you intended.

I'm not active enough on LW to be able to accuse anyone here of being idle, either in given particulars or in general. I intended more of an expression of curiosity about where the stated objectives make contact with past experience than to make a lazily hidden indictment :)

It is, of course, in a trivial way

Practicality is usually in some sense "trivial", not so? Is there any sense in which the word implies complexity or subtlety?

It could be a case where heritable should not be confused with not environmental.

There's also an element of instrumental rationality quite beautifully captured there, shifting the focus from ideation to concrete action by setting a certain waterline for what should be regarded as worth attention.

Similar to the Latin Acta, non Verba.

It's a principle of practicality - which category strikes me as largely overlapping HP's home territory.