I've been reading up on physicalism both from the stanford encyclopedia article and a book on physicalism by Daniel Stoljar. I'm now well aware of the difficulties in defining physicalism let alone whether it is true but do the same definitional problems also plague other ontological definitions of ontological substances? Such as mental or abstract substances.

Edit: To better clarify, I remember one common method to define physical objects is through the method of via negativa by contrasting it to what it isn't, a physical object is something which isn't a mental object. Further a good starting point for what is a physical objects comes from our preconceptions or intuitions regarding them, we think a mind isn't physical or mathematical objects but a table is. Finally, I must mention the theory conception of physical objects either with future ideal physics or current physical models being used to define what we mean by physical objects.

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    Could you describe more explicitly which definitional problems you have in mind, and how they come up. There are many of them, and different authors take different perspectives. It would help to address your question more pointedly.
    – Conifold
    Oct 21, 2019 at 3:24
  • Every object is defined by extracting it from the whole so the 'via negativa' is always the basis of its definition.As you say we can define a physical object as not a mental one, but we know of no such object. To define a physical object as non-mental would be Realism so it's a definition carrying a lot of baggage. The problem for physicalism.is that it requires a definition for objects that falls down in metaphysics. The definition employed embodies an ad hoc assumption, and it comes back to bite when we attempt to develop this initial conjecture into a theory. . . . .
    – user20253
    Jan 22, 2020 at 12:41

2 Answers 2


Let me throw my two cents here. I am not claiming to have answers to your questions. I have just some remarks of methodological nature and some references that you may find useful.

I think that you can only speak about precise definition (or lacks of one) of some entity in the context of some fixed system S. So when discussing notions of a physical/abstract/mental object you should do it only within certain S. Let me explain.

Suppose that you built a rigorous (or even formal) system S of (some part of) knowledge. The utility of having such a system is I think evident - without such system we are mixing various meanings, get involved in circular reasoning and get confused all the time. Science is some sort of approximation of such a system, but rather far from ideal. It is not a critique because science's main concern is to clarify or organize our knowledge, but to expand its limits.

Note that in process of building S you want to construct clear terminology. The basic methodological observation is that you have to start from some notions that are undefined and hence primitive. That is precisely the idea Plato expressed in Theaetetus:

Socrates: Let me give you, then, a dream in return for a dream:—Methought that I too had a dream, and I heard in my dream that the primeval letters or elements out of which you and I and all other things are compounded, have no reason or explanation; you can only name them, but no predicate can be either affirmed or denied of them, for in the one case existence, in the other non-existence is already implied, neither of which must be added, if you mean to speak of this or that thing by itself alone. It should not be called itself, or that, or each, or alone, or this, or the like; for these go about everywhere and are applied to all things, but are distinct from them; whereas, if the first elements could be described, and had a definition of their own, they would be spoken of apart from all else. But none of these primeval elements can be defined; they can only be named, for they have nothing but a name, and the things which are compounded of them, as they are complex, are expressed by a combination of names, for the combination of names is the essence of a definition. Thus, then, the elements or letters are only objects of perception, and cannot be defined or known; but the syllables or combinations of them are known and expressed, and are apprehended by true opinion. When, therefore, any one forms the true opinion of anything without rational explanation, you may say that his mind is truly exercised, but has no knowledge; for he who cannot give and receive a reason for a thing, has no knowledge of that thing; but when he adds rational explanation, then, he is perfected in knowledge and may be all that I have been denying of him. Was that the form in which the dream appeared to you?

By the way, Theaetetus is exactly the dialogue, where Plato discusses the knowledge.

Next I think that there are some reasonable criteria for constructing S.

  1. Consistency - S should be consistent.

  2. Expressibility - S should be designed to explain fixed set of input data. Ideally it should be complete i.e. explain the totality of intersubjective data available.

  3. Economy - the collection of primitive notions of S should be as small as it is possible with regard to expressibility. That is just an invocation of Occam's razor.

These three conditions impose certain limitations on the choice of primitive notions of S.

Now there are two famous classics in modern philosophy which discuss and rigorously construct certain systems of knowledge (or at least attempt to do so). They are chiefly focused on defining notions (so called constitutional systems) and choosing appropriate collection of primitives. These are:

  1. The Logical Structure of the World by Rudolf Carnap
  2. Structure of Appearance by Nelson Goodman

Moreover, they both attempt to do it from phenomenalist/nominalist perspective. There is also less known but great idea of reism coming from Warsaw-Lvov school of philosophy.


I was thinking about your question for a while and aside from previous metaphilosophical remarks, there are some other (rather vague) observations that are better suited to address your original question.

There was (and still is) an intellectual temptation among philosophers (Descartes is a prominent example, Berkeley is another, Chalmers is one of the most prominent examples nowadays) to consider mental objects as primitive ones. It comes from the strong intuition that we have access to the outer world mediated through our impressions, thoughts, whereas on the other hand we have direct access to our inner-self. These entities are precisely what we call mental objects in our colloquial language. I can't say that there is a similar intuition with respect to physical objects (consider electrons, curvature of spacetime, wave functions), but that is just my opinion. Nevertheless it is not unjustified to assume that mental objects are more primitive cognitively than physical ones.

Among abstract objects there are mathematical ones. Since all physical models constructed so far are expressed in terms of mathematical objects, it seems that in order to give a satisfying definition of physical object one needs first to define mathematical objects or accept them as primitive. This is another serious objection to physicalism c.f. Susan's Schneider article. It also supports a view that abstract objects are more primitive than physical.

I think that those are some reasons why defining physical objects is a more serious issue, than giving a precise definition of mental or abstract.

  • Rather intriguing answer. Oct 25, 2019 at 17:36
  • If that is compliment, then thank you very much. Otherwise I am sorry for bothering you.
    – Slup
    Oct 26, 2019 at 9:29
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    No, you gave a really good answer that really was more of a meta-metaphilosophical analysis of philosophy. Oct 26, 2019 at 20:23

Stoljar presents physicalism as having a definitional precision problem, but this is fundamentally not true. Stoljar is correct that one if one presses any definition of physicalism hard enough, then it will break under the load. But this is a common problem for ALL ideas in philosophy -- per Munchausen's Trilemma, NO justification of any view can EVER be sufficient! Is the Münchhausen trilemma really a trilemma? See my answer.

Stoljar sees the softness of definitions as a problem for physicalism, because he is wedded to an analytic approach to doing philosophy. And analytics -- REQUIRES precise definitions. But Quine demonstrated over a half century ago, in Two Dogmas of Empiricism, that we can NEVER have sufficiently precise definitions to do analytic philosophy in the pure form that Stoljar presumes: https://www.theologie.uzh.ch/dam/jcr:ffffffff-fbd6-1538-0000-000070cf64bc/Quine51.pdf.

Your question presumes, incorrectly, that it was the definitional problems of physicalism that lead Stoljar to reject it. Stoljar eventually reverts to an empirical rather than analytic mode of evaluating physicalism, which allows him to conclude that physicalism is not true. https://www.amazon.com/gp/customer-reviews/R13R2OUNXMIN6H/ref=cm_cr_dp_d_rvw_ttl?ie=UTF8&ASIN=0415452635

From this discussion, one can then infer that, yes, both mental and abstract substances will suffer from definitional inadequacies, if one tries to start one's evaluation of them from definitions, and insists on definitional precision. But a more useful conclusion is that one should NOT start with definitions, nor pretty much ever insist on definitional precision. In this respect, our philosophic inspiration should be drawn from Socrates, who devoted most of his dialogs to puncturing the delusions of definitional precision of his discussion partners. Instead one should treat philosophy as an exploratory, pragmatic, and empirical process -- as basically the precursor to the possible maturation of a future new science field.

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    It is a nice answer (+1). Although I don't agree with it completely :). Let me make a point that there are prominent scholars that consider "physical" to be devoid of substantial philosophical meaning. For instance N.Chomsky. I agree that at the bottom the notion of definitional precision is rather vague. This follows from its normative nature. Nevertheless, thank you for your interesting and enlightening answer.
    – Slup
    Jan 23, 2020 at 8:13
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    @Slup -- I believe the LP agenda, and the "logical" route to characterizing our world, has been decisively refuted by mathematical and logical pluralism. Also previously by Kant demonstrating that logic is useless in sorting between contingent possibilities in our reality. The "linguistic turn" to philosophy, was DRIVEN by the logician agenda, as precise language is needed to translate logic to our world. Chomsky is an anti-positivist but is fully enmeshed himself in the logical/linguistic error. Philosophy to progress needs to re-embrace pragmatism. I find Popper much more useful.
    – Dcleve
    Jan 16, 2022 at 16:30
  • These are good points. My remark will be somewhat Fichtean. Fichte suggested that there is a unity in logic. It manifests itself in the fact ("empirical" if you like) that logician can without doubt decide whether some content belongs to the logic. It is almost as she (the logician) is equipped with some sort of criterion which solves this decision problem. Obviously she could not be able to express this criterion explicitly, but she is tempted to believe in the unity of logic, regardless of the fact that she is perfectly aware that there are many distinct logical systems.
    – Slup
    Jan 31, 2022 at 16:48
  • @Slup Do you have a link to Fitche on logic? The neuroscience on our ability to reason supports that reasoning is not REASONABLE, but is intuitive/emotive at a pretty fundamental level: falsabeh.medium.com/…
    – Dcleve
    Jan 31, 2022 at 21:51
  • I read some of his original works. Namely: "Concerning the Concept of the Wissenschaftslehre" and I started reading "Foundations of the Entire Science of Knowledge", but didn't have time to read it completely. The idea of the unity of reason (and science) is, as I can tell, the main ingredient of Fichte's philosophical agenda.
    – Slup
    Feb 9, 2022 at 16:40

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