shrI shankara has given the entire teaching of the Upanishads in one verse which is known by the name eka shlokaprakaraNam literally meaning one verse.
किम् ज्योतिस्तव भानुमानहनि मे रत्रौ प्रदीपादिकम्
स्यादेवम् रविदीपदर्शनविधौ किम् ज्योतिराख्याहि मे ।
चक्षुस्तस्य निमीलनादिसमये किम् धीर्धियोदर्शने
किम् तत्राहमतो भवान्परमकम् ज्योतिस्तदस्मि प्रभो ॥
Kim jyotiH tava bhAnumAnahani me rAtrau pradIpAdikam
syAdevam ravidIpadarshana vidhau kim jyotiH AkhyAhi me|
chakshustasya nimIlanAdisamaye kim dhIrdhiyo darshane
kim tatrAhamato bhavAnparamakam jyotistadasmi prabho ||
(Guru:)Is it your light that shines as the sun in the daytime and as the lamp in the night time?Let it be.Which light shines when your eyes are closed? (which light illumines the mind?) Which light illumines my intellect? You are the absolute light that illumines the mind;
(Disciple:)That absolute light am I.
In this verse, again, light is used to bring out the illumining nature of Atma. Just as light helps us know things in the empirical world, so also, Atma is the illumining principle in the absolute sense; that ever shining Atma, I am. This is the import of the verse.
Let us analyze the verse step by step:
1. In the absence of sun, there is darkness all around in the world. This darkness is ‘known’ to me. We can perceive darkness, i.e. absence of light. How? The eyes coupled with the mind help us to perceive darkness.
2. The sun rises spreading its rays around the world
a) It is this star, which on rising in the morning makes the world available, visually, to beings.
b) It is the rising of the sun which makes all living beings get to work with their daily routines.
c) In the absence of the sun, we are unable to perceive objects.
3. Again, during night time, we need a light for us to be able to carry on our transactions. We use a lamp for this purpose.
In the above three instances, we are using the eye as a medium to perceive darkness, light and objects. When we see darkness, we appreciate the absence of light. We are also not able to see objects clearly. In the presence of light, we perceive objects. At this point it is important to note that when I see an object, what I am actually seeing is the object plus light. Suppose, I switch on a light in a dark room; a book lying on the table is now perceived clearly. So, when I see the book, what I am actually seeing is the book and the light reflected by it. There are two factors involved. We, by default only pay attention to the object and not to the light. But it is because of the light that the object is perceived. Remembering this, we can move forward to the next section of the verse.
4. Now, when we close our eyes, the mental world is 'visible' to us; meaning, we are aware of our thoughts, emotions, feelings, worries, pleasures etc. When the eyes are closed, there is no visible light, as such. Then how are my thoughts illumined?
It is the Atma by whose presence the inert mind functions. Not only does it function, I am also aware of the modifications of the mind. I do not need any other light to light up my mind and its functions.
5. Again when my eyes are closed, all the reasoning and rational thoughts of my intellect are known to me.
It is the presence of Atma which illumines the functions of the intellect.
All the modifications in my mind and intellect do not require a lamp outside. They are illumined i.e. ‘known’ to me through this inner ‘light’ which is Atma.
6. The Atma is the luminous principle, illumining the otherwise inert mind and intellect. It is by its presence that the mind and intellect gain sentiency. This mind acts as a medium with which the organs of perception (eyes, ears, nose, tongue and skin) and organs of action (hands, legs, speech, organ of excretion and organ of procreation) function.
Though it is the mind which appears to be behind the functioning of the organs; it is actually the Atma, since mind is made luminous because of Atma. Atma is not performing an action but its mere presence provides sentience to the mind.
7. In the wakeful state the mind helps us function in the world. In the dream state, a dream world is created with the help of the mind and, the dreamer I interact in this dream world just as the waker I interact in the waking world. In the dream world the sun and the lamp of the waking world are missing, yet, my dream, and my dream world are known to me.
8. In deep sleep, I am not aware of anything. There is absence of duality. There is also ignorance of outside world. Though the mind is resolved in deep sleep, there is a subtle thought functioning, which cognizes the absence of duality. Even though, the mind is resolved, it is illumined by the Atma. The fact that I get up and say “I slept peacefully, I did not know anything” is a proof that I had an experience of ‘absence of everything’ in sleep. There can be a recollection of experience only when there was collection of one in sleep. Atma illumines the experience of deep sleep. Do I need a sun or lamp for this to happen?
It is the consciousness that is present in all the three states of waking, dream and sleep. Atma is in and through all the three states.
Simply put, the darkness in the waking world, the sunlight, the lamp, the objects in the world are all illumined by the mind. Is the mind the ultimate light? No. The mind and intellect, themselves are inert but are illumined by Atma. The dream state, a product of the mind is also illumined. By what? By the Atma, the illumining principle. Atma also illumines the deep sleep state. Once I get up I know that I slept blissfully, and was unaware of everything. I am aware that I was unaware of the waking world. How? Only due to the presence of Atma. Atma illumines the presence and absence of all. Everything is known and unknown due to the absolute light, Atma.
This Atma, the existence principle is the supreme light. Then who am I? I myself am that Atma which is the illumining, witnessing principle in whose presence the waking dream and deep sleep states come and go. It is the nature of the Atma to illumine. It is not an action done by it. Atma is not a doer. It alone is. In its presence everything shines. That absolute light am I.