Hi Steffano. Nice to see you here again!
I do not wish to offend the good professor, but this seems be a case of too much thinking and not enough experience. Thinking needs to be grounded in reality.
As several people have pointed out, we need to be careful with our definitions. Fortunately the suttas give us good guidance. A self is defined as something that is neither impermanent nor suffering. Moreover, it is something we should be able to control.
Let’s see how this works from an experiential point of view. A state of deep meditation, specifically a jhāna, is powerful not just because it gives us the ability to focus with great clarity on the most detailed aspects of existence, but because the path into jhāna is itself very instructive. When you emerge from jhāna, you know that you have seen the three characteristics directly. Certain aspects of your experience that existed prior to jhāna – the body, the senses, the will – were completely gone for a long period of time. It is this cessation which is the true mark of impermanence. Further, you realise that the absence of these things is intensely pleasurable. You have no doubt that the five senses and the will are torturers that are best abandoned. They are suffering. You would prefer never to leave jhāna, but unfortunately that is not an option. Then there is the nonself aspect. In jhāna you cannot even access the five senses and the will. It is not just that they are gone, which professor Repetti argues is not enough to label something as nonself. It is the fact that they are completely beyond you. No access is possible.
Imagine a tree. Most people would say a tree cannot be their self because it is external to us. A tree is not endogenous to a person. Yet any given tree is still an experience and as such one might just be able to make a case that it is a self. In fact, a tree is a much better candidate for a self than the will as I have described it above. A tree is always accessible; you just have to look in its direction. Even if you are far away, you can potentially see the tree with the help of an electronic device at any time of your choosing. Not so the will in jhāna. From this point of view, you would be better off taking the tree as your true self than the will. Yet we can probably agree that it is absurd to take a tree a one’s true essence.
Of course, the above is still in the realm of thinking. In the end only experience of these things will be fully satisfactory. Nonetheless, thinking about this in the right way may at least make it plausible that nonself can be seen through direct experience.