 Importantly, for Yablo, conceivability, thus understood, helps address a pressing challenge from the post-Kripkean literature, namely the issue of modal error. This is the issue of explaining what the sources of our erroneous modal judgments are (see, e.g., Bealer 2004 and Yablo 2006). While modal error can in principle affect both a priori and empirical modal reasoning, Yablo focuses on the challenge to conceivability as a guide to possibility that arises from Kripke’s cases of the necessary a posteriori. Not being aware of the fact that Hesperus is Phosphorus, I might find it conceivable that one exists without the other; thereby I conclude that it is possible for Hesperus to exist without Phosphorus. But that is a modal error. So how does Yablo’s conceivability do us any good? For him, conceivability-based evidence is positive defeasible evidence of possibility. When properly understood as specified by  (CON),  conceivability confers prima facie justification for the modal belief in question. Yablo draws an analogy with perception, which is also a fallible source of knowledge. He argues that in the case of perception, the possibility of error and illusions does not keep us from assigning prima facie justification for beliefs based on perception. Similarly, the possibility of modal error in conceivability does not undermine the thesis that conceivability provides prima facie evidence for possibility. Although for Yablo there is a difference between conceivability and perception that makes the former provide more doubtful evidence, the difference is not a matter of fallibility or likelihood of error. Rather, the difference for Yablo lies in the fact that we have a much better story of how perceptual error comes about compared to modal error (see also van Inwagen’s 1998 discussion of modal skepticism based on the analogy with vision, as well as Bealer 2004 for discussion of the sources of modal error). This is also true of debates about intuition as a source of justification (see Weinberg 2007). However, while Yablo’s notion of philosophical conceivability can answer criticism based on the possibility of modal error, he concedes that more work needs to be done to produce an exhaustive account of modal error that will best guard us against it.