Lao Sun Tao

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I don't know if you're familiar with Peter Kingsley, Rupert, he's a scholar who claims that some of the pre-Socratics (such as Parmenides and Empedocles) were basically non-dual mystics (and priests, healers and magicians) who practiced and taught something like this method (which he reckons is the original meaning of "common sense," a term later re-purposed by Aristotle to denote what we normally call common sense these days), as well as a more inward-directed practice called "incubation," often used by sick people to help their healing (lying down in a dark place and giving up - picture a dark cave with maybe a dozen people spaced out, lying down, and a few priests/doctors moving around quietly, keeping an eye on things), but also used as practice preliminary to the practice of common sense. With common sense, the idea was to become aware of all the sensory impressions from the 5 modalities, gathered together, as revealing a single sense (hence "common"), consciousness as such - first in sessions set apart, then eventually constantly, in everyday life. There are surely similarities between this and Dzogchen/Mahamudra practice, and the post-Satori practices in Ch'an/Zen too. (I think it's all basically the same across cultures because our construction, the way we're built, is cross-cultural.) The only known biographical detail about Parmenides' teacher is that "he taught him silence" - a phrase that, understandably, would be baffling to a scholar or an academic philosopher, so it's just passed over without much comment in the literature! (Interesting side-light, he reckons the same tradition, though lost in the West after a few hundred years, eventually infused the Sufi tradition via Egypt.)