  A new type of procedures, called protective measurements, has been proposed
by Aharonov, Anandan and Vaidman. These authors argue that a protective
measurement allows the determination of arbitrary observables of a single
quantum system and claim that this favors a realistic interpretation of the
quantum state. This paper proves that only observables that commute with the
system's Hamiltonian can be measured protectively. It is argued that this
restriction saves the coherence of alternative interpretations.
