Any quantum state must be a square integrable function in order for it to be allowed. Is there any reason that this should be so? As far as I am aware, other $L^p$ spaces can have norms defined on them in a similar way to the norm of $L^2$. Is it just experimental fact that the probability of a position measurement, say, is given by$\int |\phi (x)| dx$, or is there some more foundational reason to restrict quantum states to $L^2$.
I apologize if I am unclear, I feel like I may be missing something fundamental here.