Parallel to the development of the concept of Brahman as Universal Essence was that of the Soul or Atman also as being the Universal essence. The Atman was in like manner  shown as being both ubiquitous and immanent. This thought derived its inspiration from the Rig-Vedic concept of the Cosmic Person from whose eye emerged the Sun, the Moon from his mind, fire from his mouth, the wind from his breath, the sky from his head, the earth from his feet etc. Thus the Cosmic Person was seen as projecting into the forces of nature and the world.

A further extension of this thought in the Upanishads was to create a correspondence not only to the world in general but to the individual as well. Thus fire now entered the mouth of a person and became speech, wind entered the nose and became breath, the Sun became sight in the eyes, the moon became his mind and so on. While on the one hand creation was shown as emerging from the Universal Essence, on the other the created forces were shown  as producing the faculties of man. Thus the microcosm and the macrocosm were connected.

From the Cosmic Person analogy also arose the thought that the Universal Essence was a Great Soul of which the individual soul was an aspect (Ansha). Thus in the Brihadaranyaka Upanishad it is stated:

”Atman is the person in the earth and the person in the body… in fire and in speech; in wind and in breath; in the Sun and in the eye… in truth and truthfulness; in humanity and in the human; in the Self and in the self.”

The idea of the  immanence of Brahman had a cosmic magnificence while that of the extension of the Universal Soul at the core of beings as individual souls, was even more extraordinary with profound spiritual implications. It was philosophically and ethically elevating and deeply significant for the future course of Indian thought and spirituality. The Brihadaranyaka Upanishad clearly defines the emerging insight:

”That same thing, namely, this self, is the trace of this All; for by it one knows this All. Just as  ( if it were)a footprint” 

Again in the Svetashvatara Upanishad the same theme is reiterated:

” with the nature of this self, as with a lamp, a practitioner of Yoga beholds here the nature of Brahman”

The cosmic Brahman as an idea and the subtle Atman concept then began to be connected as we see in the Chandokya Upanishad : ”Who is our Atman? What is Brahman? Finally the two concepts merged and Brahman and Atman became synonymous. Thus the Brihadaranyaka Upanishad cleary states:

‘Verily, that great unborn soul, undecaying, undying, immortal, fearless is Brahman.’

And again in the Svetasvatara Upanishad it is affirmed:

‘The Soul which pervades all things…this is Brahman.’

This grand union of two complimentary concepts implied that the unitary cosmic realism of the first was now one with the innermost spiritual essence of the self and the not-self, the great spirit of a supersoul. Furthermore the linkage between the individual self and the Great Self was the ultimate step to an authentic Monism as in the resounding declaration of the Chandokya Upanishad”

‘Tat tvam asi’ – ‘That thou art’