Ramanuja, the eleventh and twelfth century Tamil theologian who systematized Vishishtadvaita, agreed with Shankara that reality is ultimately one and that Vedanta's non-dualism is correct, but broke sharply with him over what that oneness means. Shankara's Brahman is nirguna, without attributes, and the differentiated world of souls and matter is Maya, an appearance that liberating knowledge dissolves. Ramanuja's Sri Bhashya, his commentary on the Brahma Sutras, argues at length that this reading cannot be squared with scripture's own plain sense, which repeatedly describes Brahman as possessing real, positive qualities and as genuinely related to a real world, and that calling the entire universe of ordinary experience an illusion empties devotion, ethics and scripture itself of coherent meaning. His alternative, qualified non-dualism, vishishtadvaita, holds that Brahman is one but not undifferentiated: souls and matter are Brahman's real, eternal attributes, related to it as a body's parts are related to its self, distinct in name and function yet incapable of existing apart from the whole they compose.
This body-self analogy, sharira-shariri-bhava, is the school's central technical move. Just as a person's body is entirely dependent on and controlled by that person's self, yet remains genuinely distinct from it in nature, souls and the material world are Brahman's body, wholly dependent, wholly pervaded and controlled, yet really existing as themselves rather than as illusion. The world is not something Brahman merely appears to become; it is something Brahman genuinely possesses and inhabits, which lets Ramanuja affirm non-dualism, one ultimate reality, without denying the reality of anything the world actually contains.