When you close your eyes, start watching your breath, and random thoughts start popping up.
You will them to stop, but they don't.
At some point you realize... those thoughts aren't you. They can't be, as your intent/will is that they do not arise and they pop up regardless. They're conditioned arisings, and they belong to no one.
The internal monologue in your head isn't you thinking, it's thoughts that are perceived in your body's voice, because you think that they're you, that they represent your self/soul.
Try changing the voice to any other voice, and immediately it's just thought (not self).
Same goes for feelings, for perceptions, for the body, and eventually for all forms of consciousness/awareness itself. It all goes eventually (as being thought of as "self"), seen as belonging to no one and arising/passing due to conditions, causal interactions with the other aggregates internal and external.
There is thinking, no one thinking
There is hearing, no one hearing
There is seeing, no one seeing
In thinking, just thoughts
In hearing, just sounds
In seeing, just forms, shapes and colours.