As Sankara tries to explain the appearance of the world in the light of illusory perception, he and his followers discuss the nature of perceptual error very elaborately, particularly because the explanations of such error offered by other schools make Advaita view of the world inconclusive.
The Mimamsakas altogether deny the possibility of error in perception, holding like some Western realists, that all knowledge, at least of the immediate kind, is true. If this view is correct, the Advaita position would be altogether unfounded.
The Advaidns have, therefore, to examine this view. Now, the Mimamsakas argue, as we have seen, that the so-called case of illusion, e.g. of a snake in a rope, is really not one simple kind of knowledge, but a mixture of perception and memory, and non-discrimination between the two.
Against this, the Advaitins urge the following chief points. The judgment expressing an illusory perception, this is a snake, shows that there is here a single piece of knowledge.
It may be true that the perception of the thing present ‘this’ awakens the memory of a snake perceived in the past, but if this memory did not combine with the perception to constitute one state of cognition.
But simply lay undiscriminated in the mind alongside of the perception, there would have been two judgments like, I perceive this’ and ‘I remember a snake,’ or ‘This is’ and ‘That snake was.’
The judgment ‘This is a snake’ shows on the other hand, that snake-hood is predicated of ‘This’ or the present object; and there is, therefore, a positive identification, and not merely non-recognition of difference, between the two elements, the perceived and the remembered.
In fact, without such identification, or the belief that the present object is a snake, the reaction (such as fear and running away) which follows such knowledge would remain unexplained. Perceptual error cannot therefore, be denied.
While admitting this, the Nyaya-Vaisesika school tries to explain perceptual error in a realistic way by showing that it is only an extraordinary case of perception.
In which the memory idea, for example, of a snake perceived in the past is so vividly aroused in the mind (by the perception of the similarity of the snake in the rope) that it amounts to an immediate awareness.
So, what really existed in the past (e.g. the snake previously perceived in another place) is presented to the mind now through the instrumentality of a vivid idea.
Illusion does not, therefore, show, as the Advaitins think, the possibility of the perception of an eternally unreal thing; no unreal object can ever be perceived.
The present perception of the world cannot be explained, therefore, like an illusion, without supposing a real world perceived at least in the past; and the unreality of the world at all times can never be proved. The Advaidns reject this view on the following chief grounds.
The perception, at the present place and time, of an object which existed at some other place and time is absurd. However vivid the memory-idea may be it will be an idea of that (thing perceived there in the past) and never of a (object present here and now).
So the quality of presence belonging to the illusory object remains unexplained. To hold that a memory-idea can really dislocate a real object from its own time and place and transport it to a different time and place is equally absurd.
In any case it has to be admitted that what does not really exist here and now can appear as present, and that it is also due to our ignorance of the thing (the rope) existing here and now.
Construing these facts into a consistent theory, the Advaitins hold that in illusion, ignorance conceals the form of the existing object (rope) and constructs instead, the appearance of another object.
The non-perception of the existing form is produced by different factors such as defective sense organ, insufficient light.
The perception of similarity, and the revival of memory-idea caused by the help given by facts vanishes the ignorance to create the positive appearance of an object (the snake).
This apparent object must be admitted to be present as an appearance, here and now. It is then a temporary creation (srsti) of ignorance.
This creation is neither describable as real, since it is contradicted by later perception (of the rope), nor as unreal, because it appears, though for a moment, unlike what is unreal (e.g. the child of a barren mother) which can never appear to be there.
So it is called, by the Advaidn, an indescribable creation (anirvacaniya srsti, and his theory of illusion is called the theory of the appearance of the indescribable (anirvacaniya- khyati-vada).
This view may appear as an admission of the mysterious. But every illusion does present a mystery, and fling a challenge to the unsuspecting realist and the naturalist.
Even the Nyaya-Vaisesika realist has to admit this; and he calls it, therefore, an extraordinary (alaukika) case of perception.
The explanation of the world-appearance, in the light of an ordinary illusion, as the creation of ignorance, with the power of concealing and distorting reality, is, therefore, well-grounded.
The question may still be asked, however, as to how the present world can appear unless there was the experience of a similar one in the past.
But this would not present any difficulty, since the Advaita, like many other Indian schools, does believe that the present world is only one of a beginning less series of previous worlds and the present birth is similarly preceded by a beginning less series of previous births.
Sankara describes, therefore, the process of illusory superimposition (adhyasa) as the appearance of what was previously experienced, in a subsequent locus.
He means that through ignorance we superimpose on pure being (Brahman) the diverse forms of objects experienced in the past lives.
But even if this hypothesis of a beginning-less series is not admitted, the possibility of the appearance of existence in some other form can be maintained simply on the strength of an illusory experience.
In every case of illusion the possibility of the appearance of some form of existence in place of another form of it is demonstrated a fact which clearly shows that what does not really exist now can appear as such. The appearance of the unreal as real is thus shown to be possible by every illusion.
The Advaita view of error should not be confused with that of the nihilistic Bauddha, who holds that the utterly unreal appears as the world, or with that of the subjectivist Bauddha who holds that mental ideas appear as the external world because unlike them.
Sahkara and his followers clearly state that there is always the background of pure existence (Brahman) behind every appearance, and that this ground is neither unreal nor a mere subjective idea, but existence itself.
Though the world of normal waking experience is explained in the light of illusion and as the product of ignorance like the latter, the Advaitin, we have already seen, observes a distinction between these two kinds of appearance.
They distinguish, therefore, also the ignorance responsible for the normal world by calling it the root ignorance (mulavidya), from that causing a temporary illusion by calling this latter similar ignorance (tulavidya).
Objectivity is granted by the Advaita to both the normal world and the illusory object, by admitting creation in both cases. In this the Advaita is more realistic than ordinary realists.
Where he differs from them is that according to him objectivity does not imply reality, nor does unreality imply subjectivity (a position which some contemporary American neo-realists like Holt also admit).
On the contrary, on the strength of arguments already mentioned, every object which is particular and changeful is shown by him to have a contradictory nature, and therefore, to be not real in the sense in which pure existence is.