Youve said before that Nobles were the most vulnerable to shifting economics because their tax revenue was fixed by tradition. Couldn’t they just say that their tax was a fixed percentage instead of a fox amount?

I would emphasize more their rental income than their tax income, but the reality is that both tended to be fixed by tradition. And the problem with unilaterally changing the terms is that peasants could go to law courts to challenge that change and introduce either copies of the manorial court roll that described the terms of the tenure, or even the testimony of the oldest man of the area who could testify that the terms of tenure had been such-and-such since since “time whereof the memory of man runneth not to the contrary,” they could prevail in court.

And the reason that such low-status people might prevail in court is that the whole of feudal society was based on tradition – including the nobles’ feudal contracts and their oaths of loyalty to the sovereign – so introducing innovations threatened the foundational social contract and would bring in other powerful interests on the side of tradition, whether that’s the king and his justices or the church or other noblemen.