 If \(\psi'\) and \(\psi''\) are two (orthogonal) quantum states of a system, quantum mechanics predicts that the system can also be in the state \(\psi = (\psi' +\psi'')/\sqrt{2}\) . This is the superposition principle, a cornerstone of the theory. For instance, if \(\psi'\) is the state of a live cat and \(\psi''\) the state of a dead cat, then \(\psi\) is a state which the cat is in a quantum superposition of dead and alive; the theory predicts that this is a possible state of a cat. Why then don’t we ever see cats that are half alive and half dead? That is: why don’t we see macroscopic objects in quantum superpositions?