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How to Test a (Metaphysical) Theory?

Ilmari Hirvonen

Take any theory, model, or claim that is about something. Now, it seems rather obvious that there is a distinction to be made between what is true according to the theory and whether the theory itself is true. In order to make this clearer, consider Newton’s second law: F = ma. By substituting two of the variables with quantities, we can calculate what the third quantity should be. This tells us what is true according to the law.

However, if we want to know whether the law correctly describes the movements of bodies, we need to look up from our papers and engage with the world. In other words, we must observe the things that the law is about and see if they behave as Newton said they would. In this particular case, empirical work has shown that the second law gives a close approximation of certain situations but in others one has to use relativistic mechanics.

As a homage to Rudolf Carnap, let’s call questions that address what is correct according to the theory internal questions. After all, what a theory claims to be true is a matter internal to that theory. And, following the same meter, we shall name questions that concern the truth of the theory external questions. One cannot determine the correctness of a theory by merely looking at the theory itself, it is an external matter.

It is important to note that the distinction I’ve offered here is not the same as the one Carnap made in his article “Empiricism, Semantics, and Ontology” (1950). Though, there are some similitudes – it is not totally arbitrary that I chose to use the same terms as he did. However, explicating the differences and similarities would be beside the point of this blog post. So, if your interest lies more in the history of philosophy, I recommend reading Carnap’s original text.

We can take a closer look at philosophical theories, including metaphysical ones, from the perspective of these two kinds of questions. And it’s good to keep in mind that one cannot say that a metaphysical theory is true just by looking at the theory itself. Just like determining whether Newton’s second law is correct requires more than merely examining the law on it’s own. One cannot answer external questions simply as a by-product of attending to internal questions. Something more is needed.

Now, with respect to metaphysics there is a curious discovery to be made: Metaphysical theories are supposed to be about the world, this world, the empirically accessible world. And yet it’s very difficult, perhaps impossible, to evaluate them empirically. This raises the question: why should we think that the theories are actually about the empirical world at all? Some metaphysicians could even admit that this is not what they are supposed to do. The ontologists might say that metaphysical theories concern the metaphysical, not the empirical, realm.

Well, this would be a good answer, if it wasn’t for the fact that at least some metaphysical theories clearly deal with targets located in the empirical world. Metaphysicians ask such questions as whether holes or people exist, what is the essence of dogs, when does a lump of clay become a statue, and so on. These questions, and the answers to them, are evidently about actual holes, people, dogs, and statues. So, it isn’t clear why we should think that metaphysical theories are not about the empirical world.

But all this is connected to an even larger problem, namely, the fact that there is no agreement on which of the metaphysical theories are correct or even how we should assess them. That is to say, there is no consensus about how to answer external questions about metaphysics.

One suggestion is that metaphysical theories should be evaluated on their theoretical virtues, such as coherence, simplicity, and explanatory power. This would offer a way to answer external questions on the correctness of metaphysical theories. But note that the proposal is also in itself a philosophical theory. It is a metametaphysical, a metaontological, or a metaphilosophical theory.

Therefore, here too one can separate internal and external questions. To the internal question: “What is true according to the theory?” we can answer: “The correctness of metaphysical theories should be evaluated with theoretical virtues.” But we still need to give a reply to the external question: “Is this metametaphysical theory correct?” and this requires some way to assess the metaontological position.

Some have claimed that the metaphilosophical position is true, because the same theoretical virtues are applied in science and the tremendous success of science justifies these virtues (see, e.g., Paul 2012). If it is true that also scientists utilise these virtues, then this would indeed be a strong argument for the metametaphysical theory.

But alas! Others have pointed out that the same virtues are not used in science, they are not used in the same way as metaphysicians use them, they are not able to make a difference between metaphysical theories, or they are not truth-conducive (see, e.g., Sober 1988; Bennett 2009; Ladyman 2012; Kriegel 2013; Benovsky 2014). (I’m grateful to Henrik Saarinen for helping with the references on current metaphysical debates.)

Thus, the metaphysician is left with the following conundrums: First, why can’t metaphysical theories be evaluated empirically, given that they concern the empirical world? Second, if they are not about empirical reality, what reality do they refer to? Third, why should this non-empirical world be postulated in the first place? Fourth, how should the theories be evaluated, if not empirically? And fifth, on what basis would this other way of assessing them truly say anything about their correctness?

Before dealing with these problems, we cannot give justified answers to external questions in metaphysics. And without being able to answer the external questions, the internal questions don’t appear to be that interesting. That is, before we have a solid epistemology of metaphysics there is no real point in doing metaphysics. Let’s wait and see whether one is coming.


Bennett, Karen (2009): “Composition, Colocation, and Metaontology” in Metametaphysics: New Essays on the Foundations of Ontology, David Chalmers, David Manley, and Ryan Wasserman (eds.), 38–76.

Benovsky, Jiri (2014): “Tropes or Universals: How (Not) to Make One’s Choice” Metaphilosophy 45(1): 69–86.

Carnap, Rudolf (1950): “Empiricism, Semantics, and Ontology” Revue Internationale de Philosophie, 4(11): 20–40.

Kriegel, Uriah (2013): “The Epistemological Challenge of Revisionary Metaphysics” Philosophers’ Imprint 13(12): 1–30.

Ladyman, James (2012): “Science, metaphysics, and method” Philosophical Studies, 160(1): 31–51.

Paul, L. A. (2012): “Metaphysics as modeling: the handmaiden’s tale” Philosophical Studies, 160(1): 1–29.Sober, Elliott (1988): Reconstructing the Past: Parsimony, Evolution, and Inference, Cambridge, MA: The MIT Press.

Published by Ilmari Hirvonen

PhD student at University of Helsinki working on philosophy of pseudoscience. Dabbling in some philosophy of religion in spare time.

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