David Lewis, my guess for the most important philosopher of the last half century, seems to reduce philosophers who disagree with him to saying "just because, that’s why." Consider Peter van Inwagen and Phillip Bricker.
Peter van Inwagen’s 1992 paper "It Is Wrong, Everywhere, Always, and for Anyone, to Believe Anything upon Insufficient Evidence?" was the first in the modern series of philosophy papers on the rationality of disagreement. He wrote "a polemic against what I perceive as a widespread double standard in writings about the relation of religious belief to evidence and argument":
How can I believe (as I do) that free will is incompatible with determinism or that unrealized possibilities are not physical objects or that human beings are not four-dimensional things extended in time as well as in space, when David Lewis–a philosopher of truly formidable intelligence and insight and ability–rejects these things I believe and is already aware of and understands perfectly every argument that I could produce in their defense? … I suppose my best guess is that I enjoy some sort of philosophical insight (I mean in relation to these three particular theses) that, for all his merits, is somehow denied to Lewis. And this would have to be an insight that is incommunicable – -at least I don’t know how to communicate it–, for I have done all I can to communicate it to Lewis, and he has understood perfectly everything I have said, and he has not come to share my conclusions. But maybe my best guess is wrong. I’m confident about only one thing in this area … it must be possible for one to be justified in accepting a philosophical thesis when there are philosophers who, by all objective and external criteria, are at least equally well qualified to pronounce on that thesis and who reject it. … if you grant that evidence may include incommunicable insight, can you be sure, have you any particular reason to suppose, that it is false that there are religious believers who have "insight" that lends the same sort of support to their religious beliefs that the incommunicable insight that justifies your disagreement with Kripke or Quine or Davidson or Dummett or Putnam lends to your beliefs?
Thus the one thing van Inwagen is most sure of is that he must somehow be justified in disagreeing with someone much smarter who has understood all the same communicable evidence.
In the December 2006 issue of Philosophical Perspectives, Phillip Bricker takes issue with Lewis’s famous conclusion that actuality is relative, that each possible world is real on its own terms. Bricker says that Lewis’s strongest challenge to the idea that only our world is really actual is this:
There are concrete merely possible people who are epistemically situated exactly as we are: there is no evidence that can distinguish our predicament from theirs. But then we can’t rule out the possibility that we are the merely possible people inhabiting a merely possible world.
Bricker accepts this lack of distinguishing evidence, and that usually it undermines claims to knowledge. On this topic, however, Bricker is an "epistemic chauvinist," who
holds that her beliefs may constitute knowledge even though another subject, actual or possible, with the same evidence, the same concepts, and the same powers of reasoning holds beliefs contrary to hers. … I claim to know that possibilia exist in spite of the preponderance of benighted philosophers who disagree. It’s not that I think there are any non-question-begging arguments that will force them, by the light of reason, to see the error of their ways. The light of reason, I simply conclude, shines on me and not on them.
Bricker also thinks he must be justified in disagreeing with Lewis, even if Lewis is smarter and the usual arguments favor Lewis’s side. Bricker’s and van Inwagen’s unreasoned disagreements with Lewis seem to me to be clear cases of bias, i.e., people who should admit their case is weak, and who should at least be much less certain. Beware of disagreeing with David Lewis!
(FYI, Arguments similar to Lewis’s also question how we could know that we have been conscious or that any part of us is new conscious.)