postulate and the Born rule, as we explained above, one looses the standard physical interpretation of the mathematical formalism (this will play a crucial role later on when we evaluate the ability of decoherence to solve the measurement problem).

With this quantum formalism in place, it is easy to show the discrepancy between its predictions and experience. In this regards, Zurek in [@Zur p. 4] states:

> \[A\]t the root of our unease with quantum theory is the clash between the principle of superposition – the basic tenet of the theory reflected in the linearity of \[the Schrödinger Equation\] – and everyday classical reality in which this principle appears to be violated.

More formally, consider a quantum system $S$ with Hilbert space $H$ and consider an observable $A$ with eigenvectors $\{{|a_i\rangle}_S\}$. Consider also a measuring device $M$, with ready state ${|r\rangle}_M$, which interacts with the system in the following way $${|a_i\rangle}_S {|r\rangle}_M \xrightarrow{Schrödinger} {|a_i\rangle}_S {|a_i\rangle}_M$$ for all $i$, where ${|a_i\rangle}_M$ is the (macroscopic) state of $M$ which indicates that the result of the measurement is $a_i$. That is, $M$ is a good measuring device. Then, by the linearity of the Schrödinger equation, it is clear that $$\left( \sum_i c_i {|a_i\rangle}_S 