PSR Skepticism

Here I discuss Koons and Pruss’s recent argument that skepticism about PSR leads to skepticism about whether we have any empirical knowledge at all.

Koons and Pruss present what they call a “dialectical” defense of a restricted version of PSR, aiming to show the “disastrous consequences” of rejecting it.1 The particular consequence they have in mind is “radical empirical skepticism”, in that if PSR is rejected “we cannot know that we have any empirical knowledge of the external world (either in particular or in general).”2

1 Restricting PSR

Here is the restricted form of PSR that they endorse:3

Restricted (and Non-Circular) Principle of Sufficient Reason:
If the xx’s are some basic natural facts, then there is an (actual fact) y, that is not one of nor part of one of the xx’s, and y explains the xx’s.

Natural facts are facts that are both (i) wholly particular and (ii) are not identical with or constituted by any fact concerning an entity that is unbounded and “infinite in intrinsic measure.”4

Koons and Pruss also reject (though they don’t really argue for this) the position that a natural fact (or facts) could be self-explanatory, as well as that a totality is explained so long as each member of the totality is explained.

We claim that no wholly self-explanatory fact is a basic natural fact. Equivalently, we claim that any explanation of any plurality of basic natural facts must be distinct from any member of that plurality.5

Given the version of PSR they endorse, Koons and Pruss argue that there must be at least one “supernatural” fact that can be the explanans of the totality of basic natural facts.

If we assume that all explanations of particular facts must include a particular fact, we can use this version of the PSR to demonstrate the existence of at least one supernatural fact, and so the existence of at least one concrete supernatural being.6

Koons and Pruss leave unexplored what the nature of such a being would be. Is it God? The multiverse? Spinoza’s Natura Naturata? It’s unclear. Suffice it to say that though they accept a restricted form of PSR, they reject naturalism and physicalism, since neither of these are compatible with the existence of explanatory but non-natural facts.

2 PSR & Skepticism

Koons and Pruss argue for the truth of a conditional claim – the “Central Claim”:

Central Claim:
If we can know that we have any empirical knowledge of the external world (either in general or with respect to some particular proposition), then we are in a position to know a priori that the PSR for basic natural facts is robustly true.

Before turning to evaluate the claim, let’s note a couple of important aspects. First, the claim is not about empirical knowledge per se, but about our (higher-order) knowledge of such knowledge. However, there are places where Koons and Pruss seem to suggest that it is both first and second-order knowledge that is at stake (e.g. p. 1084, 1087). It’s not clear to me why first-order knowledge would be threatened, at least not without taking on some very strong (and therefore controversial) internalist assumptions about justification.7

Here’s one argument from the falsity of PSR to skepticism about first-order knowledge.8

  1. Empirical knowledge depends on our experience being the reliable causal effect of interaction with an “external” world.
  2. If PSR is false, it is possible that some or other experience is uncaused.
  3. If it is epistemically possible that under “close” counterfactual conditions our experience is not reliably caused by the external world, then we cannot have knowledge of the external world
  4. ∴ If it is epistemically possible that PSR is false, we cannot have any empirical knowledge of the external world.

This is not a particularly compelling argument. Premise (1), while at least somewhat plausible, is not obviously true. Perhaps we can have knowledge in one-off cases even though we are not thereby reliable in the relevant sense. But even granting this premise and the second, premise (3) seems unnecessarily strong. Why think that our actual knowledge depends on being able to rule out, a priori, that some counterfactual scenario does not obtain? All of the responses available to the epistemologist in countering more standard cases of radical skepticism are available here.

Moreover, it is not clear how we are to evaluate the “closeness” of a world in which PSR is false. After all, by the author’s own lights, there is no empirical difference that we can discern between a world in which PSR is false and another in which it is true. So on their view, our evidence for which world we might be in, and thus which worlds are close or distant, is going to be the same in all relevant cases. But then we cannot determine which of the worlds under consideration is our actual one, and thus which are close or distant to it. In other words, any world in which experience is supposed to count as evidence is going to entail skepticism, since such evidence is inconclusive. But this seems to presuppose the conclusion – that our evidence is inconclusive absent the PSR.

3 Concessions to the Neo-Humean

Koons and Pruss concede that their argument fails against a neo-Humean position.

Let’s further suppose that the world’s actual laws of nature (thought of now as brute regularities that count as laws because of their belonging as axioms to the ‘‘best system of laws’’ for the Humean mosaic of the actual world) are of such a character as to ensure that all of our empirical data and epistemic links have causes. Consequently, these events must have causes in all nearby possible worlds, and the preconditions of empirical knowledge are met.

This objection is sound, as far as it goes. If Neo-Humeanism were the true and adequate account of modal reality, empirical knowledge would not depend on anything like the PSR. This is not surprising, given the extremely deflationary attitude of Humeans toward causation and explanation.9

This concession is important, because the neo-Humean position (this shouldn’t be confused with Lewis’s actual view, since there are aspects of that view that are optional for the neo-Humean) seems like one of the primary rivals to the proposed view of PSR.

There are no firm arguments brought against the neo-Humean position. There is the assertion that a causal powers ontology is the correct one, and a brief argument against the neo-Humean characterization of “closeness”.

Since Humeans deny the fundamental reality of causal powers, they ought to measure closeness of worlds solely in terms of how similar their Humean mosaic of qualities are. For this reason, a world in which the pattern of qualities in the Humean mosaic is close to that of the actual world should be relevantly ‘‘close’’ (for the purposes of sensitivity or safety), regardless of whether that world verifies the laws of nature of the actual world. Hence, there will be close worlds in which inductively supported generalizations are falsified in the subject’s immediate future. Consequently, even if it is ‘‘rational,’’ in some sense, to believe empirically confirmed generalizations, we could never know them to be true, if Humeanism were correct.

One problem with the above argument is that the characterization of the Humean mosaic itself is going to depend (epistemically) on the laws (or patters of regularity) that hold at the world. So there is no obvious way of holding the mosaic fixed while altering the laws in a way that counts as a “close” world. And even if one were to resolve this problem, there is a further one, as it isn’t even obvious which laws hold at our actual world. An indefinite variety of laws are consistent with the history of the universe up until now. So how can we specify close possible worlds with different laws when we can’t even be sure which are the laws that hold in our actual world?10



Hall, Ned, Brian Rabern, and Wolfgang Schwarz. 2021. “David Lewis’s Metaphysics.” In The Stanford Encyclopedia of Philosophy, edited by Edward N. Zalta, Fall 2021. Metaphysics Research Lab, Stanford University.
Koons, Robert C., and Alexander R. Pruss. 2021. “Skepticism and the Principle of Sufficient Reason.” Philosophical Studies 178 (4): 1079–99. doi:10.1007/s11098-020-01482-3.
Lewis, David. 1986. On the Plurality of Worlds. Oxford: Blackwell.
———. 1999. Papers in Metaphysics and Epistemology. New York: Cambridge University Press.
Lewis, David K. 1987. Philosophical Papers: Volume II. First Edition. Oxford University Press, USA.
Nolan, Daniel. 2005. David Lewis. Chesham: Acumen.
Sider, Theodore. 2011. Writing the Book of the World. Oxford: Oxford University Press.

  1. (Koons and Pruss 2021, 1079). ↩︎

  2. (Koons and Pruss 2021, 1080). ↩︎

  3. (Koons and Pruss 2021, 1083). ↩︎

  4. (Koons and Pruss 2021, 1083). ↩︎

  5. (Koons and Pruss 2021, 1082). ↩︎

  6. (Koons and Pruss 2021, 1083). ↩︎

  7. Such assumptions would include that I have to have general knowledge about the grounds of the reliability of my senses, or that knowledge of any particular fact requires that I am at least in a position to rule out any defeaters or undercutters of that knowledge (e.g. that I am a brain in a vat or deceived by an evil demon, etc.). See the discussion in §§3-4 to the effect that empirical knowledge itself depends on some form of PSR. Most of these arguments come down to whether or not the putative knower is in a position to rule the possibility of a deviant causal chain with respect to their first-order empirical beliefs. If they are not in a position to rule out such deviance, then they are not first-order knowers. Much here hinges either on considerations mentioned above or on the notion of “cause” in play. I discuss this further below. ↩︎

  8. See (Koons and Pruss 2021, 1083–85). ↩︎

  9. (Koons and Pruss 2021, 1094). ↩︎

  10. Of course, Lewis contends that there is a way of determining which laws hold – the perfectly natural laws. But this notion of “naturalness” is controversial and Koons and Pruss don’t discuss it. So I’ll ignore it here. For a sample of recent relevant discussion see (Lewis 1986, 1987, 1999; Nolan 2005, chap. 2; Sider 2011; Hall, Rabern, and Schwarz 2021). ↩︎