For me it was the natural conclusion from coming to accept a no-collapse interpretation of quantum mechanics. Before that, the ghost in the machine seemed to me like maybe it could be hiding somewhere in the spooky apparent randomness of wavefunction collapse, but if the universal wavefunction fully and deterministically describes the evolution in time of all particles everywhere, and there are no terms for “thoughts and feelings and free will” in that equation, then they are epiphenomena.