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The first part of this article is a historical and physical introduction toquasars and their close cousins, called Active Galactic Nuclei (AGN). In thesecond part, I argue that our progress in understanding them has beenunsatisfactory and in fact somewhat illusory since their discovery fifty yearsago, and that much of the reason is a pervasive lack of critical thinking inthe research community. It would be very surprising if other fields do notsuffer similar failings.

Active Galactic Nuclei and Quasars: Why Still a Puzzle after 50 years?