Interpreting Sri Ramana while also relying upon his deep study and experience, Michael James writes,

If the essential nature of something is consciousness, it must always be conscious, because nothing can ever be separated from its essential nature. Because consciousness is the essential nature of our consciousness ‘I am,’ it is conscious at all times and in all states. Similarly, because the essential nature of our consciousness ‘I am’ is also being or existence, it exists at all times and in all states.

Happiness and the Art of Being: An Introduction to the Philosophy and Practice of the Spiritual Teachings of Bhagavan Sri Ramana, p. 28

In brief, I am cit, I am sat, and cit = sat.

Exposition

1. The only thing we know for certain is that I am conscious. I can doubt the contents of experience: I may see water on the highway, only to realize that it was an illusion. In fact, I can be wrong about the content of any experience: any perception, any thought, any sensation, and so on.

But what I can’t possibly doubt, let alone get wrong, is that I am conscious. Or even just: “I am.”

If I try to conceive of a time or place where I am not conscious, that conceiving is itself an experience in consciousness. If I fear not being conscious, that fear is arising in consciousness. If I believe that I am unconscious in deep sleep (a belief that is incorrect), then that belief is occurring in consciousness. However hard I try, I can’t leave consciousness behind. After all, the one trying to leave consciousness behind is swimming in consciousness.

2. And consciousness can only be conscious: “If the essential nature of something is consciousness, it must always be conscious, because nothing can ever be separated from its essential nature.” It’s not possible for consciousness to come out of unconsciousness or to pass into consciousness. Consciousness–itself, as itself–is conscious.

3. Since consciousness is only ever conscious, what is other than consciousness cannot enter into consciousness. A physical body, being made (apparently) of matter, cannot be ‘something’ that consciousness can be conscious of.

4. Given that this is the case and given that I am conscious, it follows that I cannot be the body. If we continue to grant that the body is material, then there is simply no way for that which is material to make contact with, to touch, to interact with consciousness. But remember: the only thing I know for certain is that I am conscious(ness). So, I also know for certain that I am not the body.

5. The mind, it can be demonstrated, is nothing apart from thought and feeling (and feeling, Atmananda nicely points out, is just “intensive thought”). By reduction, we can then say: the mind is nothing apart from thought. Of course, mind arising as thought arises only intermittently. This can be easily verified during any meditation. But consciousness, being my essential nature, simply remains as consciousness whether there is thought or no thought (i.e., mind or no mind). As a result, I cannot be the mind. An additional argument comes from analyzing the three states: waking, dreaming, and deep sleep. In deep sleep, consciousness is while there is no mind. Therefore, consciousness–I–is not mind.

6. What the above does is to allow us to “purify” consciousness so that, set apart from body and mind, it can stand simply as “I am.”

I am. This is the Truth.