As I said in that thread, fighting the hypo is not polite behavior.
On the first day of a physics class, you can ask the professor to justify learning physics given the problem of Cartesian skepticism. The professor might have an interesting answer if she decided to engage you. But what would actually happen is that the professor will ask you to leave, because the conversation will not be a physics conversation, and the social norm is that physics classes are for physics conversations.
In short, the practical conversation you are trying to start is not the same as the theoretical one that was started.
As I said in that thread, fighting the hypo is not polite behavior.
On the first day of a physics class, you can ask the professor to justify learning physics given the problem of Cartesian skepticism. The professor might have an interesting answer if she decided to engage you. But what would actually happen is that the professor will ask you to leave, because the conversation will not be a physics conversation, and the social norm is that physics classes are for physics conversations.
In short, the practical conversation you are trying to start is not the same as the theoretical one that was started.