David Chalmers separated what he called the easy problems of consciousness, explaining how the brain discriminates stimuli, integrates information, and reports on its own states, all in principle tractable by ordinary physical and functional explanation, from the hard problem: explaining why any of that physical processing is accompanied by subjective experience at all, rather than occurring, as he put it, in the dark, with no one there to feel it. Property dualists argue that even a complete physical and functional description of a brain state, everything about its structure and causal role, leaves this further question genuinely open, which they take as evidence that mental properties, while entirely dependent on and instantiated by physical substance, are not identical to or logically derivable from physical properties, however tightly correlated the two turn out to be. This is meant to be a lighter commitment than full substance dualism, since it posits no separate soul-stuff and accepts that mental properties depend entirely on a physical substrate, typically the brain, for their existence, while still resisting the identity theorist's claim that a mental property just is a physical property under another name.
Critics on the physicalist side argue that property dualism inherits a version of the old interaction problem in a new form, since it has to explain how a genuinely non-physical property could arise from, and causally matter within, a wholly physical system without simply collapsing back into either full dualism or full physicalism.