|All is Physical and Physics is All|
Physicalists say that all facts are fixed by physical facts. Whatever physics tells us there is - that's it. And physics tells us that there are no non-physical things.
This creates two questions: What are qualities and what are mental properties? If physicalism, of the strongest kind, is true, then qualities are identical to quantities and mental properties are identical to physical properties.
On physicalism of this stripe, all effects are physical and all causes are physical. If all the effects that we can observe are physical then all the causes are physical. Empirical evidence renders non-physical causes for physical effects implausible. The measurable quantitative properties of the physical causally explain what is qualitative and are nothing more than the quantitative. The quality of a color is identical to the quantity of saturation, hue and brightness. The mental property of pain, likewise, just is the physical property of a neurological state.
If you meet a physicalist there are some objections you can offer.
Three Possible Objections
"I" refers to me. If I am a brain then whatever is true of I and whatever is true of my brain is identical. I can conceive of being in another body. I could exist while my brain doesn't. If it is true that it is possible that I exist when my brain does not then there would be something that is true of "I" that is not true of my brain. Therefore, I and my brain are not identical. This shows that there is at least one property that is not identical to a physical property.
Empirical observations assume the uniformity of nature. Nature works according to laws that are predictable and knowable though repeated testing. Consequently, empirical observations carry maximal warrant. How do we know that nature is uniform? We know nature is uniform by observing the uniformity of nature. In order to empirically observe the uniformity of nature we need to assume the uniformity of nature. But that means we must assume something in order to prove it. But then there is something we would have to know that Physics cannot tell us.
Cf. David Spurrett & David Papineau, "A note on the completeness of ‘physics’"