 In epistemology, a prima facie reason for a belief is a reason which justifies this belief in the absence of defeaters, i.e., of exceptional information to the contrary. Prima facie reasons have been suggested in epistemology as a means of providing weak and defeasible a priori justifications for epistemologically fundamental beliefs such as the belief in the existence of an external reality, for which a strict and non-circular a priori-justification seems to be impossible. Nevertheless, the inference from visual appearances to reality counts as defeasibly justified as long as there do not exist serious defeaters, i.e., reasons for serious doubt (cf. Williams 1996, 2001, and the SEP-entry on a priori-justification and knowledge). An example is the defeasible inference from visual appearances to beliefs concerning external reality. My awareness of my visual appearance of a tree in front of me is a prima facie reason for my belief that there really is a tree in front of me, provided that information about exceptional circumstances is absent, for example the information that I am under the influence of drugs which cause hallucinations (cf. Pollock 1986, Moser 2002). One may object that a similar kind of possible exception to the inference from visual appearances to reality is constituted by the possibility of a Cartesian demon who infuses humans with illusionary perceptions, which is one of the major skeptical counterarguments to the inference from appearances to reality. In response to this objection, several epistemologists have argued that these kinds of exceptions are not serious exceptions, and therefore, the inference from appearances to reality is defeasibly valid.