In his description of the Vijnanavada theory of perception which is strikingly like what in Western philosophy would be called subjective idealism Sankara introduces their concept of impossibility.
Accordingto this Vijnanavada, all dealings concerned with means of knowledge, objects of knowledge, and ends of knowledge become possible as subjective occurrences superimposed on consciousness; for even though an external object may exist, there can be no activity of the means of knowledge etc., unless it be through a superimposition of that object on consciousness (i.e.unless the mind or intellect is aware of it). If it be asked, "How, again, can it be known that all these processes are subjective, and that nothing but subjective cognition exists"?-the answer he (the Buddhist) gives is that, this is so because no external thing can possibly exist.
That’s a startling claim also akin to the thought of Berkeley’s immaterialism though the rationale is based on Buddhist atomic theory or what fundamentally exists, a purely metaphysical position being beyond human perceptual capacity.
"This is so because no external thing can possibly exist. When external objects are assumed, they must either be the individual atoms or an aggregate of them--a pillar for instance. Of these the knowledge of the atoms cannot be acquired through a perception of the pillar etc., for the atoms are not objects of perception.38a And a pillar etc. cannot be a conglomeration of atoms, since the (aggregates like) pillar etc. cannot be ascertained to be either different from or identical with the atoms. Thus also are to be rejected genus, (quality, action), etc. Moreover, knowledge, as it arises, has the common feature of being a mere awareness; but it displays a selective bias for individual forms,such as the knowledge of a pillar, the knowledge of a wall, the knowledge of a pot, the knowledge of a cloth, and so on. This particularization cannot be possible unless there be some peculiarity in each individual cognition itself. "
That is a long citation but interesting for its delineation of the core insight which sustains subjective idealism in its myriad forms. There is a subject, there is an object, there is internal reality and external reality. How are the two realms to meet unless one be superimposed on the other? This is the original aporia of metaphysics and there the paths of Realism and Idealism diverge. To stay with the Buddhist anfractuosity for now Sankara has the answer to the ‘impossibility’ claim, the focus of this post. He treats it as a flouting of modal direction if you like.
Buddhist : Since no object can possibly exist externally, I come to the conclusion that it appears as though it is outside.
Vedantin : This conclusion is not honest, since the possibility or impossibility of the existence of a thing is determined in accordance with the applicability or non-applicability of the means of knowledge to it, but the applicability or non-applicability of the means of knowledge is not ascertained in accordance with the possibility or impossibility (of the thing).What is known through anyone of the means of knowledge, such as direct perception etc., is possible, and what cannot be known through any one of these means of knowledge is impossible. In the case under discussion, the external things are known individually by the respective means of applicability or non-applicability of the means of knowledge is not ascertained in accordance with the possibility or impossibility (of the thing).
In other words you can’t get the results before you sit the exam. Impossibility or possibility is ascertained by an application of a criterion, a test of some kind but you cannot say that a thing is impossible because a test cannot be applied. There must be something to test and there is no testing of the inexistent. This may also be connected to the Thomist idea that the reality of something guarantees its possibility.
