  So, the phenomenon of disagreement supplies a skeptical threat: for many of our cherished beliefs. If we aren’t sheltered, then we know that there is a great deal of controversy about those beliefs even among the people who are the smartest and have worked the hardest in trying to figure out the truth of the matter.  There is good reason to think that retaining a belief in the face of that kind of controversy is irrational, and a belief that is irrational does not amount to knowledge. It follows that our beliefs we recognize as controversial do not amount to knowledge. This is the threat of disagreement skepticism (Frances 2018, 2013, 2005; Christensen 2009; Fumerton 2010; Goldberg 2009, 2013b; Kornblith 2010, 2013; Lammenranta 2011, 2013; Machuca 2013).