As I understand, nomologically possible world is a world which is governed by the laws of physics of the actual world, but doesn't that kind of entail that in all nomologically possible worlds (including ours) physicalism is true? Or is the term 'laws of physics' here referring not to the results of modern physics (i.e. theory of relativity or QM), but to something broader, allowing some metaphysical claims to be true?.

  • A world composed of the laws of physics as scientists understand them would have undefined behavior at the horizons of black holes, due to the contradiction that exists between QFT and GR in such circumstances. Do we want a world with undefined behavior? That seems distinctly ugly.
    – labreuer
    Mar 14, 2014 at 15:44
  • I was hesitant to write "theory of everything (given there is one)" instead of "theory of relativity or QM", so I went with the latter. And still, I don't think that the laws of physics care whether we have a unified mathematical description of them, anyway :)
    – user132181
    Mar 14, 2014 at 16:31
  • Ahhh, but are we given physicalism, or does it merely match our current laws of physics?
    – labreuer
    Mar 14, 2014 at 16:34
  • I don't understand you, could you paraphrase your question?
    – user132181
    Mar 14, 2014 at 16:35
  • Why do we think physicalism is true? Is it not due to our current conceptions of the laws of physics?
    – labreuer
    Mar 14, 2014 at 16:42

1 Answer 1


The short answer is no. Physicalism is the thesis that everything is exhaustively describable and explainable in the language of fundamental physics, or in "higher-order" language, such as the language of chemistry, biology, etc. that is theoretically reducible to the language of physics. Whether physicalism is true or false is a difficult philosophical question.

Here's one thing to say though: if physicalism is true, it's probably just a contingent truth, not a necessary one. Here's why. A sentence is a contingent truth if and only if there is at least one accessible possible world in which that sentence is false. The relevant sense of possibility here is nomological possibility. So, consider all those world that have exactly the same physical laws as the actual world.

Now, suppose in one of those possible worlds there is a lonely ghost made up out of some goo that physics just cannot ever explain. We know that the ghost can't interact with any of the physical stuff--this would entail a violation of the laws of physics, but the mere fact that it couldn't interact doesn't mean it can't exist. If there is such a ghost in that world, physicalism is false. This means, that even if physicalism were true in the actual world (i.e. even if there aren't any ghosts in our world) then it is merely a contingent truth.

All this argument relies on is the idea that if one conceive of such a ghost in a world nomologically identical to ours, then it is possible that such a ghost exists. This conceivability-possibility principle is controversial, but it isn't clear what better guide to possibility there is.

  • So, some world can behave in accordance to the laws of physics and still have something metaphysical (i.e. a ghost, as in your example) inhabiting it? When you say that a world is governed by the laws of physics, doesn't that essentially mean that every part of it is governed by the laws of physics? If so, then why can your ghost be a part of this hypothetical world and not be made of entities described by physics?
    – user132181
    Mar 15, 2014 at 15:29
  • The opposite of physical isn't metaphysical but rather non-physical. There could be a world full of physical stuff that acts according to the laws of physics, but there also be other stuff that isn't physical stuff and works some other way. This spooky other stuff wouldn't be "violating" the laws of physics in any way, just so long as there wasn't interaction, where ghosts were going around causing physical events. THAT would entail a violation of conservation laws.
    – user5172
    Mar 15, 2014 at 16:39
  • You might have very good reason to doubt the existence of such things as non-physical ghosts that don't interact with the physical world. But having good reasons to doubt that something actually exists isn't proof that it cannot exist, which is what is in question here. I very much doubt that John F Kennedy is still alive, although clearly it is physically possible that he could have still been alive in 2013 if he hadn't gotten shot, or sick, or had any other accidents.
    – user5172
    Mar 15, 2014 at 16:41
  • I liked you comments, good food for thought. Could you elaborate on the distinction between metaphysical and nonphysical in the edit to your answer?
    – user132181
    Mar 15, 2014 at 17:25
  • metaphysical claims are claims about the way the world is that are not physical claims. They are claims about what must necessarily be. These aren't strictly speaking scientific claims, because they aren't known by observation. Nevertheless, they are supposed to be truths about the way the world is. Many scientifically minded people find metaphysical claims spooky and strange, so these people say that a claim looks metaphysical when they mean that it looks like superstitious nonsense. (And superstitious nonsense is unfortunately what you find in the "metaphysics" section of a bookstore.")
    – user5172
    Mar 15, 2014 at 17:34

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