In his wonderfully entitled book The Vindication of Absolute Idealism pub. Edidburgh U.P 1983, Timothy Sprigge writes:
From our point of view and doubtless in effect from Bradley's, we interpret his contention most suitably as follows. We can form no conception of an intuitively and coherently fulfillable sort of an unexperienced reality; this being so, the proposition that there is such a reality is incapable of being a literal truth, for there simply no real judgement in the offing to be true.pg.112
What of the Advaitins with their vritti or mental modification? The mental modification is presented to the saksin or witness. Now this sounds mighty like representationalism or scientific realism and I have seen it interpreted as such by a teacher of Advaita who has a background in science; as though Locke were speaking in Sanskrit. He sees it as a matter of psychology.The tree in the yard is evidently not in my head so what is in my head? There is neuronal traffic and that appears to him to be a sound candidate that can represent the interests of the real tree. A nice picture but it simply isn't what Advaita is saying. It draws down the mind/body conundrum of how this cortical activity can be identical with consciousness. This does not arise in Advaita as there is no mind/body division.
The exceptionally succinct Vedanta Paribhasa of Dharmaraja Adhvarindra (ca.17th.C) frames the problem in a way which echoes the preamble to the Brahma Sutra Bhasya of Shankara (ca 8th.C.) Asks Dharmaraja: On what is the perceptuality of the object based? Unity is the answer, the substratum of the vritti and that of the object is one and the same pure consciousness. The object is a form of limitation or limiting adjunct, upadhi of pure consciousness. To simplify, what the object is in a gross way, the vritti is in a subtle way.
So where does the unknown object/ajnatasatta come into the picture and where does it fit in to the ontological picture which I have limned. In a curious way the answer has a resemblance to the view of Berkeley that God was minding the shop. All cognition is according to the Advaitins an unveiling of the ignorance which had covered the object of that cognition. The nescience that covered that object did not annihilate the being of that object. We by our cognition do not grant being to anything. The unity of being which underwrites perception continues whether the object is perceived or not. When we perceive that object simultaneously with its perception is the knowledge that it was previously unknown. Obviously this is non-empirical knowledge. One might say that the order of apprehension was (a) the object as previously unknown and, (b) the object now apprehended by a valid means of knowledge. So all the while that it was out of sight and out of mind the object was sheltering under the umbrella of being. For Berkeley this pure being, pure consciousness is identified with God and thereby the 'books in the cupboard' do not wink out when no one is thinking of them.
The object is joined into an ontological structure in the case of Advaita but in the purely epistemological schema of Immaterialism the object lacks a connection. God seems to be an ad hoc fix to placate common sense.