One challenge to David Chalmers' account of panpsychist consciousness is that it is untestable. If you argue that consciousness is everywhere (so goes the objection) then no observation can disprove your theory; therefore, it is not a sound theory.
Is this a valid objection? Chalmers is arguing that consciousness is a primitive feature of existence like charge or mass, that dimensional analysis by the four received basic units (charge, mass, distance and time) cannot in any combination "get us to" experience. One manifestation of mass is gravity. It is continuous throughout the universe; it is everywhere. Can gravity not be tested? The laws surrounding gravitation certainly can be, even though there is nowhere that gravity is truly zero.
If consciousness is (at least partly) epiphenomenal and supervenes lawfully on observable patterns in the material world, then these lawful relationships can and should be tested. The powerlessness of consciousness in epiphenomenal accounts (i.e. that our consciousness is caused, but does not cause anything, and we are in effect just along for the ride) is a problem that we've been wrestling with since Descartes and before, but it is a separate one. To argue the universality of consciousness does not make it any more untestable than gravity.
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