Criminal Minds may have changed many of its characters, but one thing that stuck around is its use of quotes at the beginning and end of an episode. Curiously, the first episode, “Extreme Aggressor,” actually has four quotes used throughout. Gideon recites each one, finishing the episode with: “Nietzsche once said, ‘When you look long into an abyss, the abyss looks into you.'”
This quote became something of a Criminal Minds slogan, as it’s reused in two later episodes. In episode 100, actually entitled “100,” Hotch begins the episode saying a longer version, “He who fights with monsters might take care, lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you.” Then, at the end of the season nine episode “200,” Jennifer “J.J.” Jareau (A.J. Cook) recites Gideon’s version.
It’s a bit of a dark quote to be tied so heavily to the show, but hey, Criminal Minds never claimed to be lighthearted.