How to understand "has, so far, been unascertainable" in this sentence?

This comes from "The history of western philosophy" by Bertrand Russell.
"Like theology, it consists of speculations on matters as to which definite knowledge has, so far, been unascertainable." What confuses me is "has, so far, been unascertainable", what is the subject of the has here? And how to understand as to which?