The main idea of the advaita (non-dualistic) Vedanta philosophy as taught by the @S'a@kara school is this, that the ultimate and absolute truth is the self, which is one, though appearing as many in different individuals. The world also as apart from us the individuals has no reality and has no other truth to show than this self. All other events, mental or physical, are but passing appearances, while the only absolute and unchangeable truth underlying them all is the self. While other systems investigated the pramanas only to examine how far they could determine the objective truth of things or our attitude in practical life towards them, Vedanta sought to reach beneath the surface of appearances, and enquired after the final and ultimate truth underlying the microcosm and the macrocosm, the subject and the object. The famous instruction of @S'vetaketu, the most important Vedanta text (mahavakya) says,
"That art thou, O S'vetaketu." This comprehension of my self as the ultimate truth is the highest knowledge, for when this knowledge is once produced, our cognition of world-appearances will automatically cease. Unless the mind is chastened and purged of all passions and desires, the soul cannot comprehend this truth; but when this is once done, and the soul is anxious for salvation by a knowledge of the highest truth, the preceptor instructs him, "That art thou." At once he becomes the truth itself, which is at once identical with pure bliss and pure intelligence; all ordinary notions and cognitions of diversity and of themany cease; there is no duality, no notion of mine and thane; the vast illusion of this world process is extinct in him, and he shines forth as the one, the truth, the Brahman. All Hindu systems believed that when man attained salvation, he became divested of all world-consciousness, or of all consciousness of himself and his interests, and was thus reduced to his own original purity untouched by all sensations, perceptions, feelings and willing, but there the idea was this that when man had no bonds of karma and no desire and attachment with the world and had known the nature of his self as absolutely free and unattached to the world and his own psychosis, he became emancipated from the world and all his connections with the world ceased, though the world continued as ever the same with others. The external world was a reality with them; the unreality or illusion consisted in want of true knowledge about the real nature of the self, on account of which the self foolishly identified itself with world-experiences, worldly joys and world-events, and performed good and bad works accordingly. The force of accumulated karmas led him to undergo the experiences brought about by them. While reaping the fruits of past karmas he, as ignorant as ever of his own self, worked again under the delusion of a false relationship between himself and the world, and so the world process ran on. Mufti (salvation) meant the dissociation of the self from the subjective psychosis and the world. This condition of the pure state of self was regarded as an unconscious one by Nyaya-Vais'e@sika and Mima@msa, and as a state of pure intelligence by Sa@mkhya and Yoga. But with Vedanta the case is different, for it held that the world as such has no real existence at all, but is only an illusory imagination which lasts till the moment when true knowledge is acquired. As soon as we come to know that the one truth is the self, the Brahman, all our illusory perceptions representing the world as a field of experience cease. This happens not because the connections of the self with the world cease, but because the appearance of the world process does not represent the ultimate and highest truth about it. All our notions about the abiding diversified world (lasting though they may be from beginningless time) are false in the sense that they do not represent the real truth about it. We not only do not know what we ourselves really are, but do not also know what the world about us is. We take our ordinary experiences of the world as representing it correctly, and proceed on our career of daily activity. It is no doubt true that these experiences show us an established order having its own laws, but this does not represent the real truth. They are true only in a relative sense, so long as they appear to be so; for the moment the real truth about them and the self is comprehended all world-appearances become unreal, and that one truth, the Brahman, pure being, bliss, intelligence, shines forth as the absolute--the only truth in world and man. The world-appearance as experienced by us is thus often likened to the illusory perception of silver in a conch-shell; for the moment the perception appears to be true and the man runs to pick it up, as if the conch-shell were a real piece of silver; but as soon as he finds out the truth that this is only a piece of conch-shell, he turns his back on it and is no longer deluded by the appearance or again attracted towards it. The illusion of silver is inexplicable in itself, for it was true for all purposes so long as it persisted, but when true knowledge was acquired, it forthwith vanished. This world-appearance will also vanish when the true knowledge of reality dawns. When false knowledge is once found to be false it cannot return again. The Upani@sads tell us that he who sees the many here is doomed. The one, the Brahman, alone is true; all else is but delusion of name and form. Other systems believed that even after emancipation, the world would continue as it is, that there was nothing illusory in it, but I could not have any knowledge of it because of the absence of the instruments by the processes of which knowledge was generated. The Sa@mkhya puru@sa cannot know the world when the buddhi-stuff is dissociated from it and merged in the prak@rti, the Mima@msa and the Nyaya soul is also incapable of knowing the world after emancipation, as it is then dissociated from manas. But the Vedanta position is quite distinct here. We cannot know the world, for when the right knowledge dawns, the perception of this world-appearance proves itself to be false to the person who has witnessed the truth, the Brahman. An illusion cannot last when the truth is known; what is truth is known to us, but what is illusion is undemonstrable, unspeakable, and indefinite. The illusion runs on from beginningless time; we do not know how it is related to truth, the Brahman, but we know that when the truth is once known the false knowledge of this world-appearance disappears once for all. No intermediate link is necessary to effect it, no mechanical dissociation of buddhi or manas, but just as by finding out the glittering piece to be a conch-shell the illusory perception of silver is destroyed, so this illusory perception of world-appearance is also destroyed by a true knowledge of the reality, the Brahman. The Upani@sads held that reality or truth was one, and there was "no many" anywhere, and S'ankara explained it by adding that the "many" was merely an illusion, and hence did not exist in reality and was bound to disappear when the truth was known. The world-appearance is maya (illusion). This is what S'ankara emphasizes in expounding his constructive system of the Upani@sad doctrine. The question is sometimes asked, how the maya becomes associated with Brahman. But Vedanta thinks this question illegitimate, for this association did not begin in time either with reference to the cosmos or with reference to individual persons. In fact there is no real association, for the creation of illusion does not affect the unchangeable truth. Maya or illusion is no real entity, it is only false knowledge (_avidya_) that makes the appearance, which vanishes when the reality is grasped and found. Maya or avidya has an apparent existence only so long as it lasts, but the moment the truth is known it is dissolved. It is not a real entity in association with which a real world-appearance has been brought into permanent existence, for it only has existence so long as we are deluded by it (_pratitika-satta_). Maya therefore is a category which baffles the ordinary logical division of existence and non-existence and the principle of excluded middle. For the maya can neither be said to be "is" nor "is not" (_tattvanyatvabhyam anirvacaniya_). It cannot be said that such a logical category does not exist, for all our dream and illusory cognitions demonstrate it to us. They exist as they are perceived, but they do not exist since they have no other independent existence than the fact of their perception. If it has any creative function, that function is as illusive as its own nature, for the creation only lasts so long as the error lasts. Brahman, the truth, is not in any way sullied or affected by association with maya, for there can be no association of the real with the empty, the maya, the illusory. It is no real association but a mere appearance.