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When your reasons for believing something are justified ad hoc, you are left susceptible to further discoveries undermining the rationale for that belief.

Neil deGrasse Tyson (2010). “The Pluto Files: The Rise and Fall of America's Favorite Planet: The Rise and Fall of America’s Favorite Planet”, p.51, W. W. Norton & Company
When your reasons for believing something are justified ad hoc, you are left susceptible to further discoveries undermining the rationale for that belief.