&gt;No one denies that when a measurement is performed, the measuring system and the measured system become entangled (and the aggregate system containing both continues evolving unitarily).<p>It&#x27;s not true that no one denies this.  Adherents of the Copenhagen interpretation deny it.  They claim that measurement involves a non-unitary phenomenon called &quot;wave function collapse.&quot;<p>&gt;  I can&#x27;t quite tell exactly what you all are arguing about.<p>Exactly this.  You may not realize it, but not everyone understands that measurement and entanglement are intimately related (in fact, the exact same physical phenomenon).  In fact, some people vehemently deny it.  There are even some card-carrying physicists who vehemently deny it.<p>&gt; entanglement is not a phenomenon that can be explained classically.<p>That is certainly true (though I&#x27;ve met people who deny this as well).