As we have seen, the Upanishads are the latest, most philosophical part of the Vedas. One of the main concerns of the Upanishads is the nature of the individual's soul and its relation to the universe. Basically, the Upanishads demonstrate that the individual soul (the atman) is essentially identical with the universal soul, the Brahman. Although stopping short of claiming absolute unity, the Upanishads argue the the atman emanates from the Brahman, and thus the two share the same nature. Consequently, one may look within in order to discover the great soul of the universe.
A number of philosophical systems arose that further developed the idea of the unity of the individual with the transcendent. Of the six systems of thought that are recognized by Hinduism, two are most relevant to the question of the unity.
This first system is known as Samkhya. This system of thought argues that there are two states of reality: On the one hand, there is the reality known as Purusha. In this context, Purusha refers not to the cosmic giant of the Rig Veda, but to the Self that is eternally wise, pure, and beyond change. In Western thought we might identify Purusha with spirit or the soul, although the Hindu concept is much broader. In contrast, the second state of reality is known as Prakriti or matter. According to Samkhya, suffering stems from confusing the two states of reality. Humans tend to forget that their true Self is unlimited, eternal, and pure; instead they identify themselves with matter which is finite and changing.
Samkhya is thus a dualistic philosophy in that it maintains that all things fit into one of two categories. How can a dualistic philosophy also argue for the profound unity of the cosmos? The answer is that Samkhya argues that there is a unity between the individual human being and the eternal. The Self is identified with the Eternal.
A second system of Hindu thought is known as Advaita Vedanta. Advaita literally means "non-dualistic" and vedanta means "end of the Vedas" (i.e. the Upanishads). This system is more closely related to the thought of the Upanishads, and it is primarily associated with the philosophyer Shankara (lived in the 7th Century CE). Advaita Vedanta carries the ideas suggested in the Upanishads to a conclusion that even more closely unites Brahman and atman. While the Upanishads seem to suggest that the individual soul emanates from Brahman and the two share the same nature, Advaita Vedanta concludes that atman and Brahman are actually one. Just as the waves are not distinct from the ocean, atman is not distinct from Brahman. This system of thought is thus monistic: "Monism is the view that all reality is one unified divine reality" (Ludwig, p 13). In other words, Brahman (the world Soul or simply the divine) is all things, and all things are Brahman. By implication, this means that even the gods in Hinduism are all manifestations of the one reality known as Brahman.
While Samkhya argues that there is a material world that is finite, Advaita Vedanta argues that there really is no material world that is separate from Brahman. In fact, material life (i.e. changing and finite life) is nothing more than an illusion that prevents us from truly seeing the unity of all things with the divine. This illusion is created by a power known as Maya. It should be noted that Advaita Vedanta does not argue that the material world does not exist; rather it argues that our view of it as finite is an illusion. In reality, even what we consider imperfect and finite is nothing less than a manifestation of Brahman.