I'm asking from a mathematical aspect, please be as exact as possible. Suppose A is an observable, and B a Borel set in R (real numbers). Without further assumptions on A, I would like a general expression for the probability of measuring one of the values from B for A. The reason for my question is that it seems I'm stuck if A hasn't got any eigenvalues (like position X), and can't be associated with a wavefunction (like position X), or put in position representation. Please notice that the two identical brackets have opposite interpretations. *For example, for X, the answer would be the Lesbegues integral of [tex]\[ \left| \psi \right|^2 \] [/tex] over B.