When Ramana Maharshi was a school boy, one day, suddenly fear of death occurred to him for no reason. Thoughts like dying means the legs become stiff; lips become taut; eyes get closed, breath stops etc. came to him and due to the intensity of the feeling, the legs became stiff, lips became taut, eyes got closed and breath stopped*. But, still he could sense the vigour of the I-thought. The strong presence of individual consciousness was clear. So, while the body was dead – insentient – I sense still had a strong presence. Therefore, I sense was independent of the body. So, he was separate from the body, he thought. At that time Ramana Maharshi thought that he was under the control of some spirit, but this experience probably gave rise to Akhandakara Vritti (unbroken thought wave) as mind became focused towards the source for prolonged period and the Self Inquiry continued. In due course of time he was able to see# that the I sense was originating from pure awareness. So, this meant that he was pure awareness from where I thought was arising. In other words, he realised that he was non-individual pure awareness from where individual consciousness was coming.
*There is no mention of heart-beat here. However, since it was a real experience, it happened spontaneously rather than forming a logical sequential chain. It comes close to NDE + Self Inquiry. It is just that the description looks like a logical sequential chain.
#I sense cannot be the absolute truth because I sense is not there in deep sleep but there is existence of the person in deep sleep. Here is a quote from Upadesha Saram:
It is this existence which is the permanent reality
For even in deep sleep
Where we have no present sense of I
We do not cease to be
The above reasoning helps, but one can know this by direct experience also (when mind is Sattvic, that is when it has very less thoughts) that I sense is originating from the field of awareness. In Arunachala Ashtakam verse two Ramana Maharshi writes, “Enquiring within ‘Who is the seer?’ I saw the seer disappear, leaving that alone which stands forever”. Thanks to Sri David Godman for mentioning the point about Arunachala Ashtakam.