Warning! This article contains spoilers for Reacher season 3's episode 4.

Amazon Prime Video detective series has established that Jack Reacher follows a fixed set of principles and rules that guide his actions and moral com. He finds order and certainty even during the most chaotic and dangerous situations because his rules help him determine his next course of action.

Although he follows a long set of rules, there is one that he keeps repeating almost throughout seasons 2 and 3: "In an investigation, assumptions kill." Till season 2, it was hard not to believe that Reacher came up with that rule based on his previous investigative endeavors. However, by delving into Reacher's military backstory, season 3 finally confirms where he gets that rule from and why he never dares to break it.

Reacher Season 3, Episode 4 Reveals "Assumptions Kill" Came From Garber

Reacher Has Learned A Lot Of Lessons From The Senior Officer

When Reacher and Kohl begin their investigation into Quinn's case in season 3's episode 4, Reacher does his best to equip Kohl with everything he has learned over the years. He even repeats his iconic season 2 line, "Assumptions kill," reminding Kohl that she must rely on facts and certainties instead of jumping to conclusions. Kohl, too, agrees with him and acknowledges that it is a good rule. In response, Reacher reveals that he learned the rule from Leon Garber, his senior officer from the military police.

As the episode progresses, it becomes evident that Jack Reacher has learned a lot from Garber. For instance, when he realizes the truth about Kohl's abduction in Alan Ritchson character looks up to Garber, explaining why he diligently follows the "assumptions kill" rule.

Quinn's Return Proves Reacher's "Assumptions Kill" Rule Is Right

Reacher Unknowingly Broke The Rule While Dealing With Quinn

Reacher season 3 episode 4's flashback reveals that when Jack Reacher first caught Quinn, Quinn tried to trick him by claiming that Kohl was secretly working with him. Although Reacher trusted Kohl, he gave Quinn the benefit of the doubt because "assumptions kill." To test whether he was lying, he asked him what Kohl's first name was. After failing to answer Reacher's question, Quinn proved that he was lying about his relationship with Kohl, which helped Reacher kill all assumptions.

Reacher seems to do everything right in the flashback but ends up assuming that Quinn is dead after he shoots him in the head. This assumption costs him dearly because he later learns that Quinn somehow survived. He realizes that his bullet might have gotten lodged in Quinn's skull instead of reaching his brain, which might have helped him survive. His survival serves as a stark reminder for the Alan Ritchson character in Reacher season 3 that making assumptions can, indeed, lead to disastrous consequences in a criminal investigation.