(Reason 1) I don't believe anyone really existed before 1990, nor do I believe real, conscious people exist in poor, repressed countries. However crazy it seems, I just cannot fathom a universe where all the suffering I read about in history books really took place.
(Reason 2) Politically-charged issues like global warming and income inequality are difficult to understand because so much ostensibly credible evidence is presented by each side. I go further than simply saying it is a complex issue. While truth does exist in many places, when an issue because hotly contentious, with much to gain / lose on either side, I believe that truth literally ceases to exist.
(Reason 3) I do not believe Tyler Cowen exists, because it seems he has no supporting staff, and I do not believe one person could write and read so much while also teaching, eating, and sleeping.
(Reason 4) I don't believe the book A Death in the Family could win a Pulitizer Prize when I cannot even get a literary agent to consider my novel. Much of my world, alas, is an illusion. Either I don't exist, or that book doesn't really exist. Or perhaps the Pulitizer Prize is also an illusion?