Let us understand it....
Before understanding this let me clear some of things..
First understanding comes from direct experiences look on direct experience. If you have direct experience then it means you understand it.
Ex- A book says gravity is like if you throw up it will come down. So this is direct experience but this is direct experience of words and your reading not the direct experience of actually you throwing up and it comes down. For that you need do it and from that you will say yes this book is saying right. So information and knowledge should be very clear else you won't find this experiment helpful at all.....
Second, we will divide whole human life experience into three layers and based on that model we will prove it. This model is created by me it is person made model so to make it easy to understand it.
Third - you should read three levels of knowledge... Which I think I post already here....
Three layers of model -
Our whole life experience cannot be outside of this. I am here talking about waking state your actually life experience.
1) enviornment layer - This layer simply consist all experiences which is outside of our body. Like cars, trees, buildings etc....
2) body layer - This is the layer which we experience most. It is the perspective from which we experience enviornment layer. So it is like centre point of waking state. It consists of it own set of experience. All these experiences work for one major factor that is to survive. - heart, skin, eyes, etc....
Every experience in this state showing towards one purpose only. That is to survive.
3) mental layer - This plane is considered to be one of the most powerful one. We feel it near our head side. It's not in our head but we feel it towards our head. Just like gravity feel on earth but it have literally no physical shape. So brain is physical but mental plane isn't. This plane consist set of experiences of - intelligence, thoughts, thinking, creativity... Etc.....
We can clearly see that in our waking state of life experience we actually experience only these three layers.
Now let's begin experiment of oneness on these layers which shows the truth of it.
First layer that is enviornment.
Now ask yourself this question and try to see not only just ask questions but see also.
Is you present the experiencer present in environment? And when did experience of environment start? Does it come before or after experiencer?
You will see that no experience of this layer of environment doesn't come before or after experincer. It comes with experiencer. You cannot able to find any distance between both of them. There is a distance between you and rock but that distance itself is experience and experiencer is already experiencing it. So now tell me? Did you see any distance between both of them?
So you can see there is no distance between both the experience and experiencer. So they said to be one. Also nothing comes after or before both comes at same time. It's just our attention where it goes. Attention comes and goes. Not oneness.
Repeat same experiment for rest of two layers. You will see that there is no real distance between any of these layers and experincer.
So they are one. Just like hand and body are one because they are attach. We say that hand and body is one right? Similarly we say experiences and experiencer is one. This knowledge of knowing that there is no real distance between them and whenever you doubt this idea then take example of hand and body to clarify this concept.
Do this experiment on every layers and see that.
But point to be noted both are one both are same things but they both perform different functions. One function is to see and other function is of change itself into different experiences.
Just like hand functions is different from whole body together. But when they put together they function as whole. Similarly when experiencer and experiences put together they functions as one. I.e. oneness.