Figuring out what something IS -- can be quite a challenge. We still don't know what gravity is, despite living with it for millennia.

The challenge to non-physicalists to define the non-physical is one that cannot be met, as Hempel's Dilemma spells out. One cannot define "physical" in a way that prohibits the things that physicalists want to deny (souls, spirits, Gods, causal minds) without the definition being either useless/untestable, or false.

A good way to approach this question is to start with epistemology -- how does one develop knowledge? Definitions are a poor way to develop knowledge. Much better is observation, then model fitting. This is what empiricism does and is something most physicalists claim to ascribe to. So -- what can we learn by considering empiricism?

1. Empiricism starts with observations, which for us humans, means experiences.
2. Inference from observations involves the use of reasoning, and logic, as well as model formation. All of these involve abstractions.
3. The inference TO the existence of the physical, therefore requires the prior acceptance of experiences, and of abstractions.
4. These two other sorts of things -- experiences and abstractions, therefore are actually prerequisites to inferring the physical, and are not logically entailed to be physical themselves.

Karl Popper's 3 worlds considers the best model of our world is that here are three types of things, those with space and time properties (matter, mostly), those with just time properties (experiences, consciousness), and those with neither space nor time properties (abstractions).

If the physical is matter, then in Popper's terms, the non-physical would be worlds 2 and 3.

As with the Hempel's Dilemma critique of the definitions of physicalism/physical -- definitions of Popper's 3 worlds will be difficult to nail down with precision. Treat them as a useful approximation.

Figuring out what something IS -- can be quite a challenge. We still don't know what gravity is, despite living with it for millennia.

The challenge to non-physicalists to define the non-physical is one that cannot be met, as Hempel's Dilemma spells out. One cannot define "physical" in a way that prohibits the things that physicalists want to deny (souls, spirits, Gods, causal minds) without the definition being either useless/untestable, or false.

A good way to approach this question is to start with epistemology -- how does one develop knowledge? Definitions are a poor way to develop knowledge. Much better is observation, then model fitting. This is what empiricism does and is something most physicalists claim to ascribe to. So -- what can we learn by considering empiricism?

1. Empiricism starts with observations, which for us humans, means experiences.
2. Inference from observations involves the use of reasoning, and logic, as well as model formation. All of these involve abstractions.
3. The inference TO the existence of the physical, therefore requires the prior acceptance of experiences, and of abstractions.
4. These two other sorts of things -- experiences and abstractions, therefore are actually prerequisites to inferring the physical, and are not logically entailed to be physical themselves.

Karl Popper's 3 worlds considers the best model of our world is that here are three types of things, those with space and time properties (matter, mostly), those with just time properties (experiences, consciousness), and those with neither space nor time properties (abstractions).

If the physical is matter, then in Popper's terms, the non-physical would be worlds 2 and 3.

As with the Hempel's Dilemma critique of the definitions of physicalism/physical -- definitions of Popper's 3 worlds will be difficult to nail down with precision. Treat them as a useful approximation.