That's a commonsense definition and I think most people would agree with it. But when people speak of things being non-physical, what often seems to be intended is the idea that there is another order of being beyond the merely physical; an order that may be even be thought to be independent of the physical, and I can't see why this would not amount to a dualistic hypothesis.

why can't we experience the world as it is in itself? — Noble Dust

I don't know; maybe we can; but the question is whether we can know that we can; and I'd have to say 'no' to that.

The 2nd time you use "physical" above do you mean physical as in "an object of physics" like you do the 1st time you used the word? Because physical can mean different things — bloodninja

Yes, apparently 'physical' can mean different things; and in fact that is just what I'm suggesting by saying that if "physical' is defined as that which can be understood in terms of physics, then animals must be thought to be non-physical beings.

Fundamental to what? Terms are defined in the context of specific theories or models. So, if such a model or theory fails to give us something good, then some other might. — Πετροκότσυφας

Fundamental to experience. Likewise, theories and models themselves exist only within experience.

Right. I was merely responding to the argument above regarding the circularity. Yes there are ambiguities everywhere...

I don't know; maybe we can; but the question is whether we can know that we can; and I'd have to say 'no' to that. — Janus

Well, "we" might not know that, but maybe there are some that do know that. The Buddha, the Christian mystics. My view includes the possibility that they know that.

That's what I'm saying.

Being physical means
1) to exist in space and time and
2) to participate in causal relations.

This is a somewhat standard definition imo.

I realize I'm confusing the terms. I see it like this: experience -> theories, models

(obviously that's incomplete, it's just within the terms we were discussing).

If things which cannot be understood in terms of physics are non-physical, then animals must be non-physical, since biology cannot be reduced to physics. Also what do you think is meant when it is said that things are immaterial? — Janus

Many physicalists will of course acknowledge that you don’t study organisms through physics, but they will nevertheless insist that ultimately all of the constituents of organisms are physical. This is often put in terms of ‘supervenience’ - but the upshot is that biology emphatically can be understood in terms of physics, even if we don’t understand the details yet.

The term ‘immaterial’ is rarely used in modern English, it is more characterstic of pre-modern philosophy. You will find it in Platonism, philosophical theology, and so on (except for in the sense of meaning ‘immaterial to the point’ etc.)

Usually theists say that God has the second property but not the first one. Or dualists say the same for the mind.
Objects without properties 1. and 2. are called abstract objects.

So it is a matter of belief whether you think 2. is needed or not in the definition.

Obviously, someone who thinks that this is meaningful and true, would also hold that biology is reducible to physics. — Πετροκότσυφας

Yes, they probably would; and I would not agree with them. Whether a comprehensive account of biology may, sometime in the future, be reducible to the language of physics is an open question; but I doubt that it can, it seems impossible at present, even in principle, because the language of biology is the language of teleology and intentionality.

but the upshot is that biology emphatically can be understood in terms of physics, even if we don’t understand the details yet. — Wayfarer

How could we know it is possible if we "don't know the details yet"?

Can you give an example of something which does not conform to this definition?

You will find it in Platonism, — Wayfarer

You mean in modern English translations of Plato, don't you?

Numbers, shapes, ideas like determinism or freedom.

What kind of existence do numbers, shapes and ideas have outside our thinking them, and their temporal and spatial instantiations in nature?

Agree.

Some dualists argue that mental processes are not physical in the same way God is not physical.

@Janus I dont know. It depends on who you ask.

How could we know it is possible if we "don't know the details yet"? — Janus

Don’t ask me, ask Daniel Dennett.

OK, I thought you were affirming it.

This is Hempel's dilemma:

Naturalism, in at least one rough sense, is the claim that the entire world may be described and explained using the laws of nature, in other words, that all phenomena are natural phenomena. This leaves open the question of what is 'natural', but one common understanding of the claim is that everything in the world is ultimately explicable in the terms of physics. This is known as reductive physicalism. However, this type of physicalism in its turn leaves open the question of what we are to consider as the proper terms of physics. There seem to be two options here, and these options form the horns of Hempel's dilemma, because neither seems satisfactory.

On the one hand, we may define the physical as whatever is currently explained by our best physical theories, e.g., quantum mechanics, general relativity. Though many would find this definition unsatisfactory, some would accept that we have at least a general understanding of the physical based on these theories, and can use them to assess what is physical and what is not. And therein lies the rub, as a worked-out explanation of mentality currently lies outside the scope of such theories.

On the other hand, if we say that some future, "ideal" physics is what is meant, then the claim is rather empty, for we have no idea of what this means. The "ideal" physics may even come to define what we think of as mental as part of the physical world. In effect, physicalism by this second account becomes the circular claim that all phenomena are explicable in terms of physics because physics properly defined is whatever explains all phenomena.

The way Quine defined existence seems legit. — Meta

I haven't read Quine. Can you summarize his definition of existence?

Thanks, that's a nice exposition of the problem.

Something like that: To be is to be the value of a variable.

It means that mental processes have the second property but not the first.

Edit: maybe it is better to say minds instead of mental processes.

What kind of existence do numbers, shapes and ideas have outside our thinking them, and their temporal and spatial instantiations in nature? — Janus

They have no extra-mental existence. We apply those concepts to extra-mental things.

Physical: Of, or pertaining to, particles.
Mental: Of, or pertaining to, mind.

On the one hand, we may define the physical as whatever is currently explained by our best physical theories, e.g., quantum mechanics, general relativity. Though many would find this definition unsatisfactory, some would accept that we have at least a general understanding of the physical based on these theories, and can use them to assess what is physical and what is not. And therein lies the rub, as a worked-out explanation of mentality currently lies outside the scope of such theories.

This strikes me as verging on a Straw-Man. Does anyone really claim that everything can be explained by GR and QM (+ other branches of physics)? That is a far cry from the more reasonable position that everything is subject to the laws of physics.

Do we have a (fully) worked out theory of anything? With respect to mentality, isn't it more reasonable to claim that, when we have an explanatory theory, whatever it is, mentality will be subject to physical laws just like everything else?

This strikes me as verging on a Straw-Man. Does anyone really claim that everything can be explained by GR and QM (+ other branches of physics)? That is a far cry from the more reasonable position that everything is subject to the laws of physics. — tom

And the laws of physics are?