Weak emergence has qualities that arise from the fundamental features of the parts and the rules that connect them. For example, the shapes made by flocks of birds can be reduced to simple local interactions among the birds.

Strong emergence has qualities that cannot, even in principle, be reduced to the parts and their rules. These qualities are genuinely novel and bring powers that are not found in the constituents alone.

Strong emergence is like mixing two chemicals in a lab and, instead of producing a new compound, discovering an entirely new fundamental force of nature. Consciousness, in particular, seems to lack any physically grounded ontology. While this is a divisive claim, it is hardly original. Physicalists who appeal to weak emergence have not yet shown—nor may they ever be able to show—that consciousness is physically emergent. If strong emergence is to be taken seriously, it must be framed in a way that avoids looking like something from nothing, which would be indistinguishable from magic.

As of now, the physicalists have to demonstrate weak emergence. Failing that, we cannot dismiss strong emergence so that we don't close the investigative and theory making space.