I entirely agree - it's very hard to show causation when any given correlation could be the side effect of a near-infinite supply of other factors.
Nonetheless, it is not a logical fallacy to suggest that there are causative relationships in the market. They're there! They're just hard to prove. The best we can do is theorize, then see how well our theories predict the future. That's the whole basis of analysis.
Difficulty clearly correlates strongly with price. It's not direct due to all the other factors I mentioned, but there is a causative relationship.
Post hoc ergo propter hoc. I am going to isolate the relevant sentences and bold the key part so you can read and hopefully understand since you missed it the first time.
Different individuals value the same things in a different way, and valuations change with the same individuals with changing conditions. . . .The impracticability of measurement is not due to the lack of technical methods for the establishment of measure. It is due to the absence of constant relations.
We are talking about economics with human action and not physics. But I will concede that despite the complexities and
unknowables we can know something and from that something derive
tendencies.