But that all his arguments, though otherwise intended, are, in reality,(from Hume’s Enquiry re Berkeley:Enquiry)
merely sceptical, appears from this, /that they admit of no answer and
produce no conviction./ Their only effect is to cause that momentary
amazement and irresolution and confusion, which is the result of scepticism.
Hume as an idealist was himself on quaking ground so he may have been looking to his own defence. Is this central notion that external reality is at best an inference from inner experience quite so bafflingly irrefutable? Shankara has in my view good counters to the classical moves. In this case the position he impugns is that of Buddhist absolute idealism which shares a family resemblance to Mediate Realism or Representationalism or Phenomenalism etc. This epistemological virus is constantly morphing and it is likely that it may become current again. Philosophers are great sheep and it takes only a few notable thinkers to espouse a position to make it respectable.
Shankaracarya and Absolute Idealism (from B.S.B. II.ii.28)
Vijnavadin: Well, I do not say that I do not perceive any object, but all that I hold is ‘that I do not perceive anything apart from the perception.
Vedantin: Yes, you do speak like that, since you have no curb) to your mouth; but you do not speak logically, for something other than the perception has to be admitted perforce, just because it is perceived. Not that anybody cognizes a perception to be a pillar, a wall, etc., rather all people cognize a pillar, a wall, etc. as objects of perception. And it is for this reason that all people understand those others (viz the Buddhists) as really assuming the existence of an external thing even while they deny it by saying, “That which is the content of an internal awareness appears as though external”. For they use the phrase “as though” in the clause “as though external” just because they too become aware of a cognition appearing externally in the same way as is well known ‘to all people, and yet ‘they want to deny any external’ object. Else why should they say, “as though external”? For ‘nobody speaks thus: “Vistumitra appears like the son of a barren woman”. Accordingly, those who accept truth to be just what it is actually perceived to be, should accept a thing as it actually reveals itself externally, and not “as ‘though appearing outside”.
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 a accordance with the possibility or impossibility (of the thing) What is known through any one of the means of knowledge, direct perception etc., is possible, and what cannot be 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 knowledge; so how can they be declared to be impossible by raising such alternatives as different, non-different, etc. For external things are perceived as a matter of fact. It is wrong to say that external things do not exist merely on the ground that cognition is seen to have the likeness of an object, because the very likeness of an object is not possible unless the object itself be there, and also because the object is cognized outside.
I have been down this road before a few times and different features of the terrain strike my eye each time. Here I note the element of retorsion applied to as though outside . You cannot claim likeness to a position that you think is baseless or of which no exemplars exist. Unless you first know what outside means you cannot use it as a point of similitude. If this 'outside’ is not a possible knowledge you can have nothing to say about it.
Shankara analyses this notion of possibility which is the vital centre of idealism:
Since no object can possibly exist externally, I come to ‘the conclusion that it appears as though it is outside.
He sees it as arising out of a transcendental epistemology, an account of how things must be or how knowledge as such is structured. Existing is existing for us. Outside of this no existence is possible. Shankara dismisses this account of possible existence as merely a epistemological fiat that has no connection to empirical practice. In the normal way we declare that something is possible if the valid means of knowledge are applicable to it. The special use of ‘possible’ which is ontologically prior so to speak to the empirical is not justifiable.
This rebuttal can be applied to the ‘if conceivable then possible’ line of argument that introduced philosophical zombies. The means of knowledge that could be applied to zombies that would establish their possibility as an actuality is mysterious. The zombie is supposed to have pain but to have no consciousness of pain. This seems incoherent and contrary to our normal conception of pain or the function of pain in sentient creatures. Perhaps this whole discussion hinges on the misleading closeness of the meaning of ‘conceivable’ and ‘possible’.