(9.120) Space is not Ishvara because it cannot produce anything,
And a permanent self cannot be Ishvara because this has already been refuted.

The idea here is simple. Shantideva is trying to demonstrate that the way in which people conceive of Ishvara is actually impossible. Space is the absence of obstructive contact. It cannot do or produce anything, so how can it be a creator. Likewise, Ishvara cannot be permanent, because something permanent does not change and if it does not change how can it create anything? The point of these refutations is to help us let go of grasping at and externally existent creator of all. The alternative the Prasangikas offer is mind as the creator of all.

Our goal in explaining these things is not to judge or criticize other religious views. We should never have discussions like this with other people who believe in an inherently existent God. We should leave them free to believe whatever they want. Our job is to look in our own mind and examine what we consider to be a reasonable view.

(Naiyayika and Vaisheshika) “ “Although he is the creator, Ishvara is unknowable.”

This is also something very common that we hear in modern society. It’s usually said in some sort of new-agey mystical sense of the truth is beyond our ability to comprehend it since it is so magical. Because it is unknowable, we therefore think it is something worthy of veneration. We say that whatever is divine must be beyond our limited abilities and so therefore we never think to question or try to understand, we just worship in awe.

What is the point of talking about something that cannot be known?

Shantideva refutes all of these with one simple question, what is the point of talking about something that cannot be known? If it cannot be known, then anything we have to say about it is pure speculation at best. If we can even talk about it, then that implies that there is something of it that can be known, which reveals it is not unknowable. We developed a sort of laziness not wishing to investigate further or a complacency with our ignorance about what is still unknown.  Or worse, we assent to a view that there will always be an unbridgeable gap between us and the divine.

But more profoundly, all of the Buddhist schools agree that for an object to exist it must be able to be known by mind. If it cannot be known by mind then it implies that Buddha is not omniscient, which is in contradiction with fundamental view of enlightenment.