 A problem, however, appears in quantum mechanics if we ask what the cat itself would perceive. Say the brain of the cat measures whether its heart is beating or not. The theory predicts that the brain will find either that it does or that it does not. In textbook quantum mechanics, this implies a collapse of \(\psi\) to either \(\psi'\) or \(\psi''\). In turns, this implies that no further effects of interference between these two states will happen. And this contradicts the conclusion that interference effects, although small due to decoherence, are nevertheless real. This problem is resolved by RQM by the postulate above: the way the cat, as a quantum system, affects an external system, is not affected by the specific way the heart of the cat has affected its brain. That is, the state of the cat with respect to the external world does not collapse when a part of the cat interact with another.