The reason why ignorance and knowledge are identical is because all concepts are objectivisations; enlightenment and ignorance are also identical, for both are objective concepts.
He who has lost his objective self thereby loses his subjective self, and has found his non-objectivity---which is the absence of subject and object.
Objects are neither 'empty' nor 'non-empty', not because they are or are not this or that, but because, of themselves, they are no 'thing' whatever BUT THEIR SOURCE ONLY.
The reason for comprehending the emptiness of objects is the abolition thereby of their subject, an abolition which remains impossible as long as the objects are perceived as real (since the one is the counterpart of the other, they have no independent identity). Phenomenally, therefore, they are one concept that has a dual aspect, and noumenon is its source.
Since subject can never be abolished directly via itself, the recognition of its objects as appearance only, results in the dis-appearance of itself as a supposed object functioning as their subject.
from "Open Secret" by Wei Wu Wei