Decoherence from entanglement with the environment interferes with the coherence of the supposition, so we only see the pure state as I understand Sean Carol's

Decoherence from entanglement with the environment interferes with the coherence of the supposition, so we only see the pure state as I understand Sean Carol's argument. At this point in the video below he goes over the spin states. https://youtu.be/LGtimjuA5gA?t=2545 https://youtu.be/LGtimjuA5gA?t=2545