Three levels of knowledge.
Level 1 - self - realisation.
You are probably thinking, "Right, well, I know myself." But the truth is that you don't. Just forget about these things; we will jump right to the experiment. Ask yourself this question each time: "Who am I?" every time we dive deeper and deeper into this experiment.
First question: Are you your environment? Like trees, cars, buildings, stones, etc. No, you are not. Are you your hand? This is your hand, but you are not that hand, right? So you cannot possibly be any part of your body, right? They are yours, but not you, right?
Before we jump to the next question, you must first have a clear definition of the mind. But to make it simpler, let's just say the mind is nothing but a collection of thoughts, or thoughts which you feel toward your head. Now, are you the mind? You can experience these thoughts, and you can probably manipulate these thoughts too, but are you actually these thoughts, or are these thoughts yours? So you cannot possibly be the mind, too.
Are you a personality which you think you are, or is that personality just yours? Like a person who has a personality of anger—is that angry personality you, or is that feeling just an experience? We humans generally experience our whole life within the mind, body, or personality, labeling it as "us," but are these things actually us, or are we just experiencing these things just like how a gatekeeper watches a gate?
So, who am I then? It is a bit tricky, but from this experiment, you can clearly conclude one thing: that is, you are just a watcher, and you cannot become the things which you are watching. Simply put, from this you can clearly see that in everything—from body to mind to environment—your nature of seeing never changes, and you will always feel your presence as a watcher. So you are an experiencer, and the rest of the things are experiences. The definition of self is something which is experiencing experiences.
Now, go back again with this experiment and try to see whether it is wrong or right?
Level 2 - Experiences.
This is much simpler to understand same for last one. The only part is tough to understand is the first level.
The state which is not experiencer is simply call as experinces. Even the state where you don't experience anything is also kind of experience.
Level 3 - oneness.
final realization is that everything is one. I will try to give a simpler, easier, and more logical answer: Without an experiencer, can experience exist? And without experience, can an experiencer exist? They are like two different sides of one existence; combined, both form existence as a whole. So, you are already in oneness from birth to death because experience and the experiencer are always there. One more thing: it is also sometimes called zero or nothingness. The reason behind this is that wherever you go in existence, you will always find both the experience and the experiencer. There is nothing to add in it or remove from it. This why is call zero state.
This is the start and end of path of knowledge according to spirituality. This whole three levels are division of existence into three parts and seeing them as one is final end of this path.
So to put it more simply you are a presence. I always tell become which you already are and the answer is much simpler you already are a presence from the start. It is due to our ego and enviornment of ignorant people we find out label as human or animal or some kind of personality.
My great wishes is with you all.