I've heard stream entry described as a turning about in the deepest seat of consciousness. I've also read other accounts that make me think that there is something about the experience that is unmistakable. Is that true? If one were to attain stream entry would one know it without doubt. Is it such a notable experience that it couldn't be mistaken for anything else?
Tibetan Buddhism recognizes two kinds of milestones: "experiences" and "realizations".
Experiences are all kinds of extraordinary moments one has along the way. Like the experience of non-duality: "I am what they call God, the true goal of all religions and the real meaning of love". Experiences come and go. They leave memories, but they don't stay with you in all their might. And that's probably a good thing. I don't know how I would live walking around thinking I'm God ;))
Realizations are (sudden or gradual) breakthroughs that change one's perspective in fundamental ways. Needless to say, they stay with you, although not necessarily in an explicit form, because you tend to get used to them and stop noticing. An interesting feature of realizations, they sometimes can be retrograde (like retrograde amnesia). It slowly creeps on you or dawns on you, that you have known something or have been behaving differently for a while now. It is almost like realization spreads backward in time.
By this classification, stream entry is a realization, not an experience. Because of its nature, in most cases I suspect it is a retrograde realization.
The true stream entry is unmistakable, yes -- but I don't see why someone could not just take some arbitrary experience, or even some intermediate realization, like the realization of emptiness of so-called "objects", and imagine that to be stream-entry.
What makes it unmistakable is the fact that in order to attain it, you have to run out of other options. You have to exhaust all theories, all hypotheses, all alternative routes to enlightenment. That's why you don't hesitate, because you clearly see that there is no way it could be something else.
What's funny is this nagging feeling I have, that you reading my description of it, reduces your chances of attaining it. Enlightenment is freedom from clinging (among other things), so clinging to this notion of stream-entry must be very counterproductive. This is why I'm a little leery of Theravada's sometimes excessive emphasis on bullet-point lists and precise definitions of the stages, as well as of Zen's obsession with the notion of satori. The more your mind grasps these, the further you are from the goal.
When one attains stream entry, one should notice that one has no more identity view, doubt and ritual attachments. Whether one knows that it is called the state of Sotapanna or Nibbana, depends on one's knowledge of the teaching. Take a Pacceka Buddha for example. He also attains stream entry before becoming a Buddha. But he wouldn't know the state by name.
When it comes to Stream Entry the following should happen:
- You should have seen the dependent origination first hand in both direction
- The lower fetters would have dropped
- You should know the 4 Noble Truths 1st hand
- You can see the Marks of Existence (necessary but not sufficient)
The most unmistakable of this is 1. I think when you get in and out of Pala Samapathi you can always see this happen. Even if not you would have seen the likes break with you enter your initial experience of stream entry and reconstitute themselves when you come out of it again. This is most unmistakable. 4 - you can see before stream entry at some level, 2 - if your defilements are temporarily suspended you might think you do not have the defilements, 3 - you can easily mistake theoretical knowledge with real experience. Understanding 1 will lead you to:
- know dependent origination 1st hand where you can see the links breaking when you get the Pala Samapatti and reforming when you come out of it
- know the 4 Noble Truths 1st hand though your understanding of dependent origination
- have unshakable faith in the Triple Gem since you have seen the the results of practicing the given path has given results which you experience 1st hand
- you will let go of your ego as you see this is a main source of misery and stress as each time a ego centered though happens in your mind you see the sensations are unpleasant leading to fabrications / conditioning
- since you have 1st had experience on what got you there you know any other practice (rites, rituals, etc.) will not get you there
"stream entry described as a turning about in the deepest seat of consciousness"
Yes, it is the turning point and never fall back.
"there is something about the experience that is unmistakable. Is that true?"
True! The experience marked is for layman (non-Arya: four noble truth if taught just like writing in the air, for cula-sotapanna just like writing on the water surface, for sotapanna just like inscribed on the large stone slab will persist more than hundred years i.e. until throughout afterlife.)
"If one were to attain stream entry would one know it without doubt. Is it such a notable experience that it couldn't be mistaken for anything else?"
Yes! If one achieve the Nibanna with your own Khanda, no doubt on Dhamma knowing four Noble Truths at the same time.