|The Definition of Human|
|Title||The Definition of Human|
|Japan Release Date||July 2002|
|U.S.A. Release Date||September 6, 2005|
|Preceded by||Chapter 11: The Two Guardians|
|Followed by||Chapter 13: Fullmetal Body|
2nd Lt. Ross and Sgt. Brosh are horrified to discover that the Elric brothers have snuck out of their hotel room in the middle of the night and rush angrily to their charges' most likely location - the abandoned Laboratory 5.
Deep within the lab itself, Edward continues his fight with the armored guard known as Number 48 and, while they initially seem rather evenly matched, Ed suddenly notices an unpleasant twinge that suggests that something has gone wrong with his automail arm. Remembering that Winry had told him his new model was less durable than its predecessor, he resolves to finish the bout quickly but soon finds himself on the run from the Slicer's increasingly vicious assault, earning a collection of small wounds as he barely maneuvers out of the razor-sharp sword's path again and again. 48, amused by Edward's antics, applauds his agility and vigor but remarks that the boy's continued blood loss and growing exhaustion will be his undoing before assuring Fullmetal that his "partner outside" is being dealt with in the same manner by the other armored guard and will not be arriving as backup. Hearing this, Edward inquires as to whether 48's ally is as strong as he, to which the Slicer replies that 66's strength is a close second to his own, but Edward laughs and comments that he has nothing to worry about in that case, as he has never managed to defeat that "partner outside" in combat even once.
True to Edward's word, Alphonse appears to have a clear upper hand against his assailant, easily defleacting all of Number 66's wild attacks, much to the guard's annoyance. But the encounter takes a sudden turn when one of Al's blows knocks off 66's helmet, revealing that the suit of armor is empty inside. With a dramatic flair, Number 66 explains that he is, in fact, Barry the Chopper - a butcher-turned-serial killer infamous in Central City for having murdered twenty-three victims before he was caught and executed. But 66 goes on to explain that his execution is only the official story; in reality, he was offered a chance to live in exchange for guarding Lab 5 and had his soul torn from his human body and bound to a suit of armor. However, 66's attempt to terrify his prey is disappointed when Al not only reveals that he, too, has a hollow armor body, but also that he grew up in a rural area back east and has no knowledge of famous murderers from the capital. Taking offense at Barry's assumption that he, too, is an executed criminal, Al explains that he brother saved his life by binding his soul to a suit of armor after his physical body disintegrated. However, having heard this, Barry cunningly suggests that Al's backstory and memories are falsehoods and that his soul is a fabrication created by the alchemist calling himself Al's brother. After all, with the actual existence of a "soul" still up for debate and his physical body supposedly having been "destroyed", what proof is there that he ever was human besides hearsay from other people? Al, beginning to doubt himself, demands to know how Barry can be so sure of his own existence under identical circumstances to which Number 66 replies - after swiftly slaughtering the soldier who has come to investigate the noise of their scuffle - that his love of killing is all he needs in order to be sure of his existence.
Meanwhile, inside the Fifth Laboratory, a clever trick perpetrated by Edward gives him an opening with which to strike his opponent and cut off his helmet, thereby separating the Blood Rune from the rest of the body and releasing it from 48's control. With his opponent clearly defeated, Edward returns the blade on his arm to normal and begins interrogating Slicer about the Philosopher's Stone. However, the talking helmet declares that the battle is not yet over as his sword plunges into Edward's side from behind. As the badly wounded alchemist turns to see that the body has risen to its feet independently of the head, 48 then announces to his young opponent that, when the Slicer was finally apprehended, the authorities discovered that the killer had actually been a pair of mass-murdering brothers. The younger Slicer brother speaks up from the body and reveals that his own, independent blood rune is further down, inside the torso. Not giving Edward time to transmute another weapon, the younger Slicer charges and, helplessly wounded, Edward feels the panic of impending death. However, in this panic, he senses a familiar feeling and remembers his encounter with Scar. He claps his hands together as the younger Slicer reaffirms that he will not give the little alchemist time to make another weapon, but instead of turning his hand on his automail or the floor, Edward thrusts it at his opponent while ducking under a fresh attack and, using the same method of Deconstruction Alchemy patented by the State Alchemist killer, blows the armor body apart at the waist. Acknowledging a true defeat, the elder Slicer asks Ed to hurry up and destroy them. Edward declares that he is no murderer, but Slicer responds that only the killing of humans counts as murder. Meanwhile, Alphonse, troubled by these new questions, is having a great deal more trouble holding 66 off. Edward tells Slicer that, since he considers his brother human, he considers them human as well. Amused by this answer - since the two brothers had been called inhuman monsters throughout their long, difficult life of crime and only now are being acknowledged as human after having their flesh and blood taken from them - the elder Slicer offers to tell the little alchemist everything he knows about the Philosopher's Stone. Asserting that he and his brother will be disposed of anyway, since they failed to eliminate an intruder, the elder Slicer remarks that - though he doesn't know enough about alchemy to give any real details about the Stone - he can at least give Edward information on the beings who created it and ordered them to guard the laboratory. But before Slicer can utter another word, his helmet is impaled by a familiar pair of long, dark blades as Lust emerges from the shadows with Envy by her side.
- This chapter marks the origin of the small scar above Edward's right eye.