That the Chinese character for sunya (emptiness) is the character for sky assures me that abstraction has a solid foundation. In the Hindu Sankhya system of 5 elements akash or space is sometimes translated as sky. When Sankara was attempting to answer the objection to his superimposition/adhasya analogy in which the attributes of one thing are transferred to another as in the coils of a rope being seen as a coiled snake, he took the example of akash. The objection took the form of saying that the analogy presupposed two sensible items being confused whereas the attributes of the Jiva, weight, complexion etc cannot be superimposed on that which is not a sensible item namely the Self. On the contrary wrote Sankara (Preamble to B.S.B.) akash/sky which is without attributes has the convex shape of a frying pan (wok) and the smudges of clouds projected on to it.
Two observations are pertinent. One is the patent misunderstanding of the role of analogy in argument. It is narrowly focused to bring out some point that its proposer regards as salient. Why then does Sankara,who understands this well, attempt to extend the analogy to answer a specious objection? Perhaps here we have the operation of a temporary approximation which will be discarded as knowledge increases similar to the method of adhiropa/apavada (attribution followed by retraction). The objector is at a level which is not very sophisticated. The important element of the teaching, adhyasa, should be retained for further reflection without being diluted by irrelevant considerations. In time it will be understood that it is the fact of the movement of attributes that is the important point and not the objects involved whether sensible or not.