This is indeed very philosophical thinking, and I would say, as far as I know, your questions are fundamental and not easy to answer.
Descartes, long ago, found in the end one possible answer: when I doubt, I know that I think, so I know that I do exist. ("Dubito, cogito, ergo sum.")
For the laws of physics: there is a lot of a priori thinking, assuming the existence and reality of time, and space, and causality (as Kant found out), but many philosophers since Plato considered that we only can see a part of reality. As a studied physicist, I do not object to that.
And already Socrates said "I know that I know nothing".
One day in Paris, I saw a man in the Metro who believed he was a cat. Really. First I thought that he was an actor, or that he made a joke, but he just had turned completely mad, coming home from work, in a business suit. He crawled on the floor and behaved like a cat. It was frightening. For him, it was clear that he was a cat. He seemed surprised that we were all shocked.
Many people think that they are smart, or that they are pretty, or that they know a lot. Where is the difference?
Manuel Contreras I, Vicente Camacho, Antonio Marin Montero, Ana Espinosa, Masaru Kohno, Antonio Picado