 Higher-order evidence was introduced above by example. We can come across higher-order evidence in a variety of ways: brain-altering conditions like hypoxia, or drugs, but also more mundane sources like bias and fatigue. Many epistemologists have discussed disagreement as a source of higher-order evidence: if an equally well-informed and thoughtful person looks at the evidence and draws a different conclusion from yours, this arguably gives you reason to worry that you’ve made a rational error (see, e.g., Kelly 2005, Feldman 2006, Christensen 2007b, Elga 2008 and much following literature). And some have argued that “irrelevant influences” on belief, such as one’s religious upbringing, can be a source of higher-order evidence as well: the realization that you would have had different religious beliefs if you had been raised in a different community, for example, might give you reason to doubt the rationality of those beliefs (see Elga 2008—see Other Internet Resources, White 2010, and Vavova 2018). It is hard to find an uncontroversial definition of higher-order evidence, as different authors understand it in different ways. But even without a general definition, we can look at cases like Hypoxia to see how higher-order evidence is importantly distinctive. (This section assumes that higher-order evidence has some first-order rational import, as this is where the distinctive features arise.)