{∅, {∅}, {∅, {∅}}} : Rage Against the Meat Grinder

... and triangles only exist in the mind ...

Towards the end of the chapter, [5] The Better Consciousness, Causes, Grounds, Confrontations, at the end of a section called, Conclusions, Cartwright touches upon something that zooms in on the nature of Schopenhauer's respectful disagreement with Kant. 

"From showing that the four forms of the principle of sufficient reason stem from our cognitive capacity and can be summarized as a single principle, it does not follow that this principle refers to some simple, single, absolute ground.  To think that this follows would be like thinking there is something like a triangle in general, something over and above equilateral, isosceles, or scalene triangles.  Although one can formulate the concept of a ground in general, just as one conceptualizes a triangle in general, there are no possible objects denoted by these concepts, which are simply empty abstractions produced by discursive thought.  To think otherwise is to be a realist, falsely believing that the concepts denote objects.  In this matter, Schopenhauer declared allegiance with the nominalist:  these concepts have no objective reference and exist only as names."

Kant ignored his own profound insight that the contingency of things is itself only phenomenal and can lead to no other than the empirical regressus that determines phenomena by referring to the thing in itself variously, as the "ground," "reason," and "intelligible cause" of phenomena. 

I would like to return to this thread when I have "time" ...

note: see There's No Such Thing As Triangles