How to Prove the External World

How to Prove There is an External World

Suppose you are doing metaphysics, working on which worlds are possible worlds. It occurs to you that no worlds can be immaterial; the idea makes no sense. Thus, you conclude that all worlds must be material. Your paradigms of the material are solid bounded objects in space. You then wonder whether the existence of such things can be proven—after all, there can be perceptual illusions. Don’t they have to be inferred from data that don’t logically entail their existence? Your metaphysics then poses a problem for your epistemology. But then, relief comes in the form of the Cogito: you can prove that you exist, and you can prove that everything that exists must be material, so you can prove that at least one material thing exists. And if one exists, why not others? That thing will have parts that may separate and recombine forming other objects. Your existence thus entails the existence of material objects, given reasonable assumptions. Your metaphysics plus the Cogitogives you an external world. Similarly, your metaphysics might dictate that all possible existences exist in space (the alternative is inconceivable): asked to prove that space and spatial objects exist, you wheel out the Cogito again—you know that you exist and that everything that exists is spatial. A necessary truth in metaphysics joins with the certainty of the Cogito to prove the existence of something that goes beyond the appearances—the real existence of things in space. It wasn’t your aim to prove the existence of an external world, but per accidens you did, thanks to the Cogito.

Sometime later you hit on another metaphysical truth: every possible world is law-governed; so, the actual world must be law-governed too. You think this because otherwise the world would be unintelligible chaos; nothing would be explicable; crazy senseless things would happen all the time (marbles turning into pigs, etc.) You might hold that every world must be subject to strict laws and nothing but strict laws, or you might relax this to require only some strict laws coexisting with non-strict laws; you draw the line at worlds consisting of nothing but non-strict laws. You might hold that causation requires strict laws (though there may also be non-strict causal laws), and that every world must include causation. But how do you establish that these laws must concern an external physical world? The Cogito won’t help you, but something close to it might: for the mind is subject to psychological laws and these are non-strict. This you know from first-person inspection. But every world must contain at least some strict laws, so there must be laws other than psychological laws; by elimination, these must be physical. Your metaphysics of causation and laws entails that there must be strict laws in every possible world, but they cannot be psychological, so they must be physical. Therefore, there must be a physical reality in every possible world, given that psychological laws are non-strict. If these laws were strict, then this argument wouldn’t work, since they could constitute the laws of the universe in which such minds exist. You know your mind exists (“I think”) and you know it doesn’t obey strict laws; therefore, it must exist in a world not exhausted by its own existence, i.e., a physical world. You can prove the existence of the external world from the lack of strict laws governing your mind plus your metaphysics. We get a kind of physical Cogito: “My mind does not obey strict laws, therefore the external world exists”. The argument works because your metaphysics allows it to, since it requires every world to contain (some) strict laws. To be concrete, I know that I don’t always act on a given desire, though I sometimes do, so there must be a strict law underlying this causal fact; and this has to be physical.[1] I can use the metaphysics of causation to prove that the external world exists—facts outside my mind. If worlds could subsist on non-strict laws all the way down, then I couldn’t argue this way, since my mind would satisfy the conditions necessary for world existence. But if worlds need strict nomological nourishment, I can argue this way. Thus, metaphysics has its epistemological uses. We are not trying to establish the existence of the external world by showing how we can infer it from our sense-data; we are relying instead on metaphysical truths about the possibilities of existence plus some first-person knowledge of the mind. It’s not about reasons for belief, good or bad, but how reality has to be.[2]

[1] This argument resembles Davidson’s argument for the identity theory in “Mental Events”.

[2] This paper should be read in conjunction with my “A New Proof of the External World”. A popular style of argument is that we know there is an external world in the way a scientist knows a theory of some part of nature, i.e., by inference to the best explanation or some such thing. The knowledge is based on sensory evidence, though not reducible to it. By contrast, I am grounding such knowledge in metaphysical necessities and introspective access.