The moment you are experiencing, the subject and object are unified. The moment you start recognizing that experience, the subject and object become separate.
When the subject and object are unified, you become the candle in front of you. You become the other person besides you. You respond immediately. There is no separation. There is no subject and object.
When subject and object are unified, you can listen. So when you hear, the subject and object are unified. If subject and object are separate, you cannot hear. Your ears heard of the sound. But you don’t. So there is distance between the subject and object, and thus the separation between the subject and object. That separation is the self. You cannot hear because of this separation, the distance that stands between the subject and object. That separated space, the distance, is the thing people mistakenly recognize as the self. People tend to fixate it. But Buddhism teaches us this state is not fixated. Inevitably, the subject and object will unify.
Buddhism teaches that there are billions of moments in one snap of fingers. So there are billions of moments during which you hear. There are billions of moments during which you don’t hear. (Human beings are very slow. So it is difficult for us to recognize this.) If you stay in the state of separation too long, you will get hit by a car. Good news is that we actually are in the subject and object unified state at many moments throughout of the day.
Whenever you spend more than necessary time dwelling your mind on something, you are in a state that subject and object are separate. They occupy your mind and leave no space for other things. If you are not in that state of separation, you always have space for other things. So when you are in a state that blocks everything else for even a short length of time, you need to recognize immediately that you are separated and it is broken.
Here let me give some examples of when subject and object are unified or not separate for a longer period of time.
When you are very busy and can only get 2 or 3 hours sleep every day, you try to save your energy by not wasting your energy in unnecessary thinking. You just do things. You are still functioning. You still know things. But you are in a state that subject and object is not separated.
Another example is in sitting meditation. You breath in. Your breath out. You are always connected to the surrounding and aware of what is going on. So you are in a state that subject and object is not separated. But when your legs are painful, you worry about whether you should release your leg or not. You hesitated about it. Then the subject and object become separated.
Above is just some theory for people who always want to know. For practice, it is easy: just put everything down, and do what you are doing now.