 Systems postulate (SP):  A quantum system \(S\) is represented by a pair \((\mathcal{O}, H)\) such that (i) \(\mathcal{O}\) is a space of self-adjoint operators on a Hilbert space, representing the observables of the system, (ii) \(H \in \mathcal{O}\) is the time-independent Hamiltonian of the system \(S\), and (iii) if \(\varrho_0 \in \mathcal{O}'\) (where \(\mathcal{O}'\) is the dual space of \(\mathcal{O})\) is the initial state of \(S\), it evolves according to the Schrödinger equation.