In terms of Relativity, I have to think this over but right now it seems to me that it is sad that the New Friesian School of Leonard Nelson seems to have diverged from Fries. [On the other hand Nelson wanted to be safe from accusations of ‘psychologism’ [note 1] that were thrown at Fries. So he kept the Friesian structure but held the categories are a priori as being not sense based and not reason based (immediate non intuitive) in a strictly axiomatic way.[And that fact of not being based on the senses is what makes it a priori thus in keeping with Kant] So you can see the motivation of Nelson. But it seems to have led to wrong conclusions. Dr. Kelley Ross shows that all one needs to reconcile Friesian philosophy with Relativity is Kant's Empirical Realism.
After all, Fries held that the categories of Kant do not have to be a priori. [Contra Kant]. Rather they can be justified in away that is not by reason nor by the senses. but by "immediate non intuitive knowledge.". And this point seems to have been missed by Nelson who held that Relativity and especially GR (General Relativity) were just not right. And in a very ironic way it was Reichenbach who held strongly of Relativity and defended it by means of dividing Kant's apriori into two. One is the normal necessary apriori not based on observation. The other is subject to modification by empirical evidence.--Isn't that exactly Fries's approach exactly?!
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[note 1] the mistake of identifying non-psychological with psychological entities. For instance, philosophers who think that logical laws are not psychological laws would view it as psychologism to identify the two]
