a fundamental property of the universe, like the laws of physcis, or is it a useful and consistent tool that only exists in our mind

Ultimately, while there is a useful distinction to be made here, I'm not sure this is the way to make it; most (all?) of what we call "laws of physics" are actually surface behaviors of more complex systems; models, ultimately, of the emergent behavior of a mathematically simpler yet chaotic and computationally dense reality. Which, when investigated, may prove less fundamental rules but rather guidelines to understanding the results of a truth you don't yet fully comprehend, and perhaps couldn't model if you did.