Philosophers frequently motivate claims as possible based on conceivability. [1] It is then natural to wonder whether conceiving is a reliable method for generating justified beliefs in possibility claims. Yablo argued [2] conceiving proposition p as possible provides evidence that p is, in fact, possible. [3] Less optimistic philosophers, such as van Inwagen, [4] claim conceiving proposition p as possible provides no evidence that p is possible. If van Inwagen is correct, philosophical arguments relying on possibility claims motivated solely by conceivability are suspect. Given how widespread such philosophical arguments seem to be, van Inwagen’s claims are worth examining in detail. 

            In Section 1 of this paper, we extract and evaluate van Inwagen’s argument for Moderate Modal Skepticism, the view that while agents have justified beliefs in some – rather commonsensical – modal claims, agents do not have justified beliefs in many other – rather philosophical – modal claims. Having outlined van Inwagen’s position, in Section 2 we note a long-standing objection to van Inwagen’s argument – that it can be generalized to undermine justified beliefs in commonsensical modal claims as well as philosophical modal claims – fails, but a related worry – that van Inwagen’s argument relies on an under-motivated distinction between basic and non-basic modal claims – does not. Two responses are offered on behalf of van Inwagen, though neither are entirely satisfying. Additionally, we note van Inwagen’s argument implausibly requires agents justified in believing a given modal claim is true in every case know the modal claim is true. Having observed costs of van Inwagen’s characterization of the relationship between conceivability and possibility, in Section 3 we examine Yablo’s well-known alternative, which does not rely on an obscure distinction between basic and non-basic modal claims, and which allows justified belief and knowledge concerning modal propositions to come apart. Since there are independent reasons to prefer Yablo’s proposal to van Inwagen’s as an appropriate analysis of the link between conceivability and possibility, and since – pace van Inwagen – Yablo’s proposal does not entail Moderate Modal Skepticism, we conclude the various philosophical arguments targeted by van Inwagen are not threatened by his skeptical thesis.

[1] (Putnam, 1980)’s super-Spartans; (Putnam, 1975)’s twin-earth; (Lewis, 1980)’s pained Martians; (Jackson, 1986)’s Mary; (Chalmers, 1996)’s zombies, etc.
[2] Cp. (Chalmers, 2002); others tie modal knowledge to counterfactual reasoning (Williamson, 2007), (Kroedel, 2017).
[3] (Yablo, 1993).
[4] (Van Inwagen, 1999).