Monday, August 26, 2019

Axiom T for physical possibility

Here is an argument for naturalism:

  1. Only states that can be described by physics are physically possible.

  2. Non-natural states cannot be described by physics.

  3. Physical possibility satisfies Axiom T of modal logic: If something is true, then it’s physically possible.

  4. So, non-natural states are physically impossible. (1 and 2)

  5. So, non-natural states do not occur. (3 and 4)

I am inclined to think (1) is true, though it is something worth pushing back on. I think (2) is close to trivial.

That leaves me a choice: accept naturalism or deny that Axiom T applies to physical possibility.

I want to deny that Axiom T is a good axiom for physical possibility. The reason isn’t just that I think (as I do) that naturalism is actually false. The reason is that I think the axioms of physical possibility should hold as a matter of metaphysical necessity. But if Axiom T for physical possibility held as a matter of metaphysical necessity, then naturalism would be metaphysically necessary. And that is really implausible.

Yet Axiom T is very plausible. What should we do about it? Here is one potential move: Axiom T holds when we restrict our statements to ones formulated in the language of physics. This escapes the implausible conclusion that non-natural states are metaphysically impossible. But holding even this restricted axiom to be an axiom, and hence metaphysically necessary, still rules out the metaphysical possibility of certain kinds of miracles that I think should be metaphysically possible. So I think my best bet is to throw out Axiom T for physical possibility altogether. As a contingent matter of fact, it holds typically for statements formulated in the language of the correct physics. But that’s all.


Atno said...

Maybe I don't get it. If something is physically impossible, that doesn't mean it is metaphysically impossible. That would beg the question in favor of naturalism, so I don't see how accepting "physical impossibility -> metaphysical impossibility" could even be remotely plausible to someone who is not a naturalist

Alexander R Pruss said...

The claim in question isn't:
a. ~(Physically possible p) → ~(Metaphysically possible p).
Rather, the claim is:
b. ~(Metaphysically possible(p & ~(Physically possible p))).

I agree that (a) would be very implausible, but (b) is at least initially compelling.

The difference between (a) and (b) is that (a) says that if something is not physically possible according to the laws of *our* world, then it does not happen at *any* metaphysically possible world. That's very implausible. But (b) says that there is no metaphysically possible world at which things that are physically impossible according to the laws of *that* world happen.

Michael Gonzalez said...

My two cents:

What does "described by physics" mean? If you mean to the point of fully explaining what happened, then I don't think living things are "described by physics"; certainly not animate living things that are self-movers. Physics describes the bodies in motion (particles that suddenly moved in that direction); but it does not explain why (namely, the gazelle composed of those particles saw a lion in the grass).

The laws of bodies in motion (physics) does constrain what the gazelle can do, but it does not explain what it does.

Heck, even on the absurd and nonsensical views involving "inputs of sensory data" and such, it still requires neurology and biology to give anything like a full explanation.

I think it all comes down to what "physical possibility" means. If it means "not forbidden by the laws of physics", then I think that includes a great deal that is not describable in terms of physics (physics has language for position, momentum, spin, charge... but it has no language at all for wellbeing, awareness, choice, etc).

Tom Hickey said...

Naturalism is a methodological assumption of scientific method. Virtually all scientists accept naturalism as a methodological assumption. There is an illogical jump from naturalism and a methodological assumption to materialism or physicalism as an ontological assumption. Methodology does not imply ontology. Category error. Adding apple and oranges. Not all scientist make this error, but many seem to do so. But I believe the implied reasoning is circular, presupposing that methodological naturalism is based on ontological materialism or physicalism.

In addition to being a useful assumption for scientific method, naturalism is also an epistemological assumption that usually boils down to assuming some form of empiricism or positivism. Therefore, it is not a useful epistemologically unless one buys into empiricism or physicalism as a world view that underlies an ideology.

Given this world view, the epistemological assumption is justified implicitly if not explicitly as based on ontology. Therefore the epistemological assumption cannot justify the ontological assumption.

There are a lot of issues with justifying the ontological assumption of materialism and physicalism, as well as the epistemological assumption of empiricism and positivism. Others have set forth arguments counting these assumptions.

A similar situation applies in economics and social science in general, where many assume methodological individualism based on ontological individualism, which again involves an illogical jump. There is no contradiction in individuals that are not homogenous (not like atoms) behaving in terms of a system in which they are elements with the relationships within the system influencing the elements, especially in a complex adaptive system like a society that is fundamentally different from physical systems owing, e.g., to reflexivity, synergy, and emergence.

In short, think model-building and assumptions like scientists do. And remember that there are no absolute criteria in a relative world. Just as assumptions are stipulated, so too are criteria.

Tom Hickey said...

@ Michael Gonzalez

Agree. Physics not only does not describe life and consciousness, it has no framework for doing so.

See for example, Erwin Shrödinger's "Mind & Matter" in "What is life?" Things have not changed much since this was written although it is now a hot topic in cognitive science and transpersonal psych.

Brandon said...

It seems to me that this issue arises from separating truth from necessity and possibility here -- i.e., you get the problem only if you assume that truth is univocal. I think it makes more sense to hold that just as we have metaphysical and physical possibility, we have metaphysical and physical truth, where the latter is a subregion of the former. Thus, Axiom T holds, but you either need to stay within the same domain (physical truth implies physical possibility, metaphysical truth implies metaphysical possibility) or recognize physical truth as a subregion (physical truth implies metaphysical truth and thus implies metaphysical possibility). The interpretation excluded (metaphysical truth implies physical possibility) gives the puzzle.

James Chastek said...

Can you say more about the "physically possible"? I'm understanding it as something like "an event which, if it happened, would occur in space-time". But then Axiom T is enough on its own to give you naturalism.

James Chastek said...

Let me clarify, I'm unclear what role the "non-natural" is playing in the argument, since from (1) and (3) alone it would follow that if an event is true then physics can describe it, and that seems to be what naturalism is.

Michael Gonzalez said...

James: Without the steps in this argument, non-natural states could be physically possible, though not describable by Physics. For example, if one takes "physically possible" to mean "not forbidden by any physical law", that would let miracles, God, immaterial minds, etc, be physically possible. Given (1) and (2), though, these would be impermissible.