The imagery of the Sun features throughout the teachings of Advaita. It appears multiple times in the Upanishads and is first found in the Vedas. But what is its significance and how does it relate to the ultimate reality of Brahman? 

The meaning Rig Veda gives us couldn’t be clearer, 

The Sun is the Self of the whole world both moving and non-moving and rises with its own effulgence in heaven, the earth and atmosphere. [1]

This dispels a literal interpretation of the Sun as simply referring to the external object, unrelated to the reality of Advaita. The Sun is used as a symbol for the Self for many reasons. One is that the Sun’s light shines down on all things like an eye in the sky. In the same way, the Self illuminates all things, being beyond all of them. We find the same understanding throughout the Upanishads. For example,

As the Sun, the eye of all the world, is not tainted with the stains in external objects seen by the eyes, so, the one internal Self of all living things is not tainted with the world’s grief, being external to it. [2]

The Sun, as the Self, is associated with the eye for its ability to provide vision and light just as the Sun does. The Brihadaranyaka Upanishad tells us that one who thinks the eye is separate from the Self does not know it, 

When he breathes, he is called ‘breath’, when he speaks, ‘speech’; when he sees, ‘eye’… They are just the names of his actions. Whoever worships one or other of them does not know… One should worship him as ‘Self’, for in that all these (actions) become one. [3]

The Kena Upanishad adds,  

What one does not see by the eye, but by which the eyes are able to see, know that as Brahman. [4]

So, the Self, Sun and Brahman are synonyms. They are the source of (and behind) all actions. The Maitri Upanishad explains that, 

The Sun is the outer self, the breath (prana) the inner self… Someone has said, ‘The one who, knowing, freed from evil, the overseer of the eye, his mind purified, based upon that, with his sight turned inwards… [5]

This means that the ‘outer’ is a symbol for the ‘inner’. The “overseer of the eye” is the Self and refers to that “by which the eyes are able to see”, as the Kena Upanishad teaches. It is known when the mind is purified, free from external attachments, where attention or “sight” is turned inwards. 

Turning inwards means tracing the awareness of the act of seeing to the source – of both the action and the sight itself. It is realising the answer to the question, “Where does the awareness of seeing come from?” 

Calling ‘breath’, prana, the ‘inner self’, is another synonym for the Self or Brahman. It is not the literal breath, but the life, spirit or vitality of all living things. As the Upanishads teach, the “Sun is breath” [6] and “the Sun is Brahman” [7].

Rather than being used in the sense of being different to the outer, it is called the ‘inner’ self because it is within all beings. With the knowledge of Advaita, ‘inner’ and ‘outer’ are relative terms, as one sees that all is Brahman and the internal-external distinction doesn’t exist. As the Chandogya Upanishad says,  

The light which shines beyond the sky, behind all, behind everything, in the unsurpassed, highest worlds, is the same that is the light within the person. [8]

This ‘light’ is the Self (as the Sun), just like how the Sun is always shining beyond the clouds, even if we cannot see it. The passage also means that the light of the Sun is no different to the light within all beings, as both are ultimately the Self which is indivisible. 

With this understanding, it is clear why the Sun symbolises the Self. The Maitri Upanishad states as clearly as Rig Veda that, 

Brahman is light and this light is the Sun. [9]

The meaning of the Sun as the Self or Brahman is littered throughout the Upanishads and is an extension of its meaning in the Vedas. By symbolising the Self, it highlights that it is not taught as simply referring to the literal Sun. Ultimately, the knowledge of the Self is seeing that it is, and is in, all things. This means anything can be contemplated as the Self as an aid for realising its nature. In this case, it is the Sun which is the chosen image that is identified as the Self or Brahman. This is merely one of the many ways that the Self is thought of. 

[1] Rig Veda 1.115.1
[2] Katha Upanishad 5.11
[3] Brihadaranyaka Upanishad 1.4.7
[4] Kena Upanishad 1.6
[5] Maitri Upanishad 6.1
[6] Prashna Upanishad 1.5
[7] Chandogya Upanishad 3.19.1
[8] Chandogya Upanishad 3.13.7
[9] Maitri Upanishad 6.3