The question is how do we know that turiyam persists in the deep sleep state?
The Mandukya Upanishad suggests that we inhabit 3 identifiable sources of consciousness that is namely and a 4th one simply called Turiyam meaning the fourth which persists in all of these 3 variable states OR is the only persistence within these three:
- Waking state
- Dream state
- Deep sleep state
Vedanta seems to hold the position that that which changes is not the ultimate reality and it makes intuitive sense to me. I also understand how consciousness or the watcher is present in the dream state and the waking state.
I always have an issue with the deep sleep part. Let us assume consciousness is not the permanent reality and an epiphenomenon of the brain just for the sake of argument. Then during deep sleep, this consciousness is "turned off" simply; and it is the memory of lost time that enables us to account for that lost time in deep sleep, after we have entered the waking consciousness. Similarly for general anesthesia when drugs simply stop all brain activity associated with experiencing anything. Why should anyone assume then that turiyam persists in the deep sleep when one cannot comment anything about deep sleep? Sounds like an assumption to me. Saying that consciousness is present in the deep sleep state but simply has nothing to illuminate can also be said as there is a deep sleep experience yet the consciousness itself doesn't exist at that point. Does it make logical sense?