Shankara is a famous advocate for a fully monist interpretation of the Upanishads and the Vedanta philosophy that developed from it. For instance:
“The difference between the individual self and the highest Lord is owing to wrong knowledge only” (1.3.19,Sourcebook of Indian Philosophy, 515). “As the rivers losing the names and forms abiding in them disappear in the sea, so the individual self also losing the name and form abiding in it becomes united with the highest person” (1.4.21, 517).
One of his great critics is the later philosopher Ramanuja, who summarizes Shankara’s philosophy as follows: “Eternal, absolutely non-changing consciousness, whose nature is pure non-differenced intelligence, free from all distinction whatever, owing to error illusorily manifests itself…as broken up into manifold distinctions – knowing subjects, objects of knowledge, acts of knowledge. And the purpose for which we enter on the Vedanta-texts is utterly to desroy what is the root of that error, i.e. ignorance, and thus to obtain a firm knowledge of the oneness of Brahman” (543). He comments: “This entire theory rests on a fictitious foundation of altogether hollow and vicious arguments, incapable of being stated in definite logical alternatives.” For instance, if Shankara were right about the utter non-differentiation of substance, it couldn’t be proven in human language, which is utterly pervaded by attention to substantial differences. And moreover, far from Shankara’s dualism being intuitively right, “all consciousness implies difference: all states of consciousness have for their object something that is marked by some difference,” such as “I saw this” (1.1.1, 543).
Ramanuja opposes Shankara’s monism with what Sarvepalli Radhakrishnan calls a “qualified non-dualism” (508). He argues, “What all these [Vedic] texts [just quoted] deny is only plurality in so far as contradicting that unity of the world which depends on its being in its entirety an effect of Brahman, and having Brahman for its inward ruling principle and its true Self. They do not, however, deny that plurality on Brahman’s part which depends on its intention to become manifold – a plurality proved by the text, ‘May I be many, may I grow forth’ (Ch. Up. VI.ii.3)” (549).
What does that mean, though? For help, we can turn to another great critic of Shankara, the early-twentieth century Indian philosopher Sri Aurobindo, who seems to me to espouse something like a “qualified non-dualism.” He insists, “It is only when we put aside all irreconcilable antinomy between Self and world that things fall into their place by a less paradoxical logic. We must accept the many-sidedness of the manifestation even while we assert the unity of the Manifested…Conscious Being…is bound neither by its unity nor by its multiplicity” (587). In practice, this means that “we can attain to the highest without blotting ourselves out from the cosmic extension. Brahman always preserves Its two terms of liberty within and formation without, of expression and of freedom from the expression. We also, being That, can attain to the same divine self-possession” (588).
Interestingly, I see real intimacies between Aurobindo’s metaphysics of creation and David Hart’s theology of the Trinity. He writes, “Christian thought has no metaphysics of the one and the many, the same and the different… The triune God…is not the high who stands over against the low, but is the infinite act of distance that gives high and low a place” (The Beauty of the Infinite, 180-181).