The Sanskrit word (or words) tattvamasi, first appearing in the Brihadaranyaka Upanishad, is usually translated into English to mean: ‘Thou art that.’* tattvamasi was and is taken to mean: ‘All (unqualified) is brahman/atman (unqualified)’, ‘I (unqualified) am brahman (unqualified)’, ‘You (unqualified) too are brahman (unqualified).’ Or, if personal need be: ‘All (unqualified) is god (unqualified).’
It is crucial to note that tattvamasi is unqualified. Hence it does not mean that some or a selected part of ‘thou’ is some or all of ‘that’. tattvamasi means ‘all of thou (warts and all) is all of that!’ And that is the extraordinary and final (i.e. at the end of Veda) ancient Indian insight (cum experience), namely pantheism that at once fully liberates (from the world as samsara) and fully binds (as an act of self-sacrifice into the world as samsara).
It is voluntary self-binding into samsara that constitutes the ultimate (meaning brahman/atman) sacrifice.**
The insight and/or experience of tattvamasi (warts and all!) turns him or her who gains that insight or experience into a true paramahansa and his or her action as that of an avatar. Consequently, since none of the great Upanishad commentators, such as Badarayana and Adi Shankara, and, lately, Vivekananda understood the foregoing, none qualified as paramahansas and avatars. They were ignorant, albeit erudite and clever, bookworms.
Clearly, tattvamasi is unqualified. It means ‘thou, warts and all, art that’. Alas, once spoken (because so experienced by a genuine paramahansa become jivanmukta), ancient Indian politically minded commentators, i.e. Brahmins, did their utmost to qualify the word. For the notion of an unqualified ‘thou art that’ opens up a right a-political can of worms. ‘Thou art that’ not only makes all ‘thou’s’ equal (or at-one) with/in ‘that’ and the means to ‘thou’, but also equal (or at-one) with/in all other ‘thou’s’ and their means to ‘thou’. That insight (or reality) was deemed politically unacceptable by both religious (i.e. Brahmin) and secular law makers because unworkable. But the unqualified ‘thou art that’ (meaning pantheism or pandeism) is the most reasonable because universally observable albeit uncomfortable explanation of the origin of ‘the world’. What it says is that all thou’s (i.e. all appearances) are full and equal participants in the whole creation process, i.e. before the universal (creation) law or dharma.
*… none of the three elements of tattvamasi, namely ‘thou’, art’ and ‘that’ have yet been clearly defined. For the notion of ‘that’ ancient Indians produced three options, namely prajapati (i.e. the father of creation), or atman, vaguely meaning ‘self’ or brahman. Specifically the notion of brahman, repeatedly declared to be unknowable, could not be reasonably described (though often suggested to mean prâna, translated as (divine) breath or life force).
**… The early Upanishads were an advanced attempt at describing the transition from infantile to mature (post 30’s) human understanding of the creation process, the latter to be applied as mature human behaviour/dharma. The first attempt had been made in the Veda and by the lawgiver Manu. (After 30) Vicarious or ritual sacrifice, that connects the infant to a yet transcendent creation ground, is now no longer meaningful, hence acceptable. From now on sacrifice means actual personal self-sacrifice, meaning that life (or appearance) itself is sacrifice. In Christianity the transition (i.e. from suck (-er) to succour, or from predator to prey, or from vicarious sacrifice to personal sacrifice) is clearly represented by the (self-) sacrificial death of an initially infantile Jesus on the Cross, and which (self-) sacrificial death every living being/appearance must emulate (to experience the joy of heaven) or be damned (in a hell of their own making).