Exactly the reasoning suggesting I might be a Boltzman brain are based on physical laws I know of but if I am only such a brain all theses physics, logic, math

Exactly the reasoning suggesting I might be a Boltzman brain are based on physical laws I know of but if I am only such a brain all theses physics, logic, math and statistics are just part of my imagination and could be totally invalid. How can I reason about probabilities of a brain existing in a universe I know nothing about? Even the concept of a brain itself would be just a mad idea.