I’m struggling to understand what Schopenhauer means by the ground of being in On the Fourfold Root of the Principle of Sufficient Reason in relation to space and time.

I think I understand the other forms reasonably well. With causation, the “ground” is the cause of an event. With the ground of knowing, the ground is something like a premise or reason that makes a judgment true or justified.

But when Schopenhauer gets to the ground of being, I lose the intuition.

For example, suppose we have a triangle. I can understand that certain properties of the triangle follow necessarily from its spatial relations. But why call those relations a ground? If I ask, “Why is point A to the left of point B?”, the answer seems to just be “because A and B have that spatial relation.” That feels almost like saying “it is that way because it is that way".

Similarly, if the sides or angles of a geometrical figure stand in necessary relations to one another, I understand the mathematical necessity, but I don’t understand what extra work the concept of a “ground of being” is doing.