The show's over-arching themes:

!![[PowerOfTrust Trust]]
Reflected in one of the series' earliest ArcWords:
->''"In the end, we're all alone and no one is coming to save you."''
-->--"[[Recap/PersonOfInterestS01E03 Mission Creep]]"

Originally spoken by Reese to Jessica in the airport flashback (where it also doubles as {{Foreshadowing}} of Jessica's fate); various references to "going it alone" or "you're all alone out here" appear in several episodes (''e.g.'' the Iraq flashbacks in "[[Recap/PersonOfInterestS01E09 Get Carter]]").

->''"You have to trust somebody."''
-->--"[[Recap/PersonOfInterestS01E02 Ghosts]]"

The counterpoint to the above, first used by Finch while trying to convince [[VictimOfTheWeek Theresa]] not to run away again; seems to show up at least once an episode. Culminates in the discussion of trust between Reese, Carter, and Fusco in "[[Recap/PersonOfInterestS01E23 Firewall]]":
->'''Fusco:''' You can't trust us, how we supposed to trust you?\\
'''Reese:''' Trust is complicated, Lionel. For example, I'm sitting in a police car with one cop who tried to murder me and another who spent six months trying to lock me up. So you'll forgive me if I take things one step at a time.

There's another counterpoint which is repeated frequently (''e.g.,'' Reese to Carter in "Get Carter," to Maxine in "Bury the Lede"):
->''"[[YouAreNotAlone You're not alone.]]"''

This becomes extremely important in "[[Recap/PersonOfInterestS03E08 Endgame]]".

!![[ILetGwenStacyDie Loss]]
Established in Reese's voiceover, the first lines in the [[Recap/PersonOfInterestS01E01 pilot episode]]:
->''"When you find that one person who connects you to the world, you become someone different. Someone better. When that person is taken from you, what do you become then?"''

All of the main characters have lost someone close to them:
* Reese lost Jessica (the voiceover is what he says to her killer when he confronts him in "[[Recap/PersonOfInterestS01E21 Many Happy Returns]]")
* Finch lost Grace and Nathan when the latter was killed in a [[MakeItLookLikeAnAccident government-orchestrated staged terrorist bombing]] and he had to [[FakingTheDead fake his death]] and [[NeverFoundTheBody loss of his body]] to keep the former out of harm's way.
* Carter's marriage failed when her husband came back from the Middle East with PTSD (revealed in "[[Recap/PersonOfInterestS03E08 Endgame]]")
* Fusco is divorced (under circumstances never fully explained)
* Shaw lost her father at age ten ("[[Recap/PersonOfInterestS03E05 Razgovor]]") and her partner Cole ("[[Recap/PersonOfInterestS02E16 Relevance]]")
* Root's StartOfDarkness was triggered by the loss of her best friend as a child.

It's true of many of the [=POIs=] as well: Elias' entire life before the series has been one complex exercise in avenging the loss of his mother. Numbers such as Dr. Tillman in "[[Recap/PersonOfInterestS01E04 Cura te Ipsum]]" and Darren in "[[Recap/PersonOfInterestS01E14 Wolf and Cub]]" are seeking revenge against someone who took a loved one away from them. Other numbers, such as Wayne Kruger in "[[Recap/PersonOfInterestS03E02 Nothing to Hide]]" are on the receiving end of revenge.

As noted on the main page, Reese is a ''very'' self-aware version of this trope:
->''"I know what happens when you take a life. You lose a part of yourself."''
-->--"[[Recap/PersonOfInterestS01E04 Cura te Ipsum]]"

The theme is also present in The Machine. It's "a system of unimaginable power" built to fight monsters, but it seems to have turned the people in the government who control it a little monstrous themselves:
->''"Everything we did was to make the world a safer place, but we strayed from the path. I admit it. I have so many regrets, but itís grown so big, and weíre so, so small. I donít know who you are or how you know all this, but maybe youíre right. Maybe Nathanís death, it is my fault. I saw the signs. I ignored them. Maybe I deserve this. "''
-->--'''Alicia Corwin,''' "[[Recap/PersonOfInterestS04E17 Karma]]"

Many episodes also question the morality and utility of vigilante action:
->''"[[BadGuysDoTheDirtyWork Maybe it's up to me to do what the good people can't.]]"''
-->--'''Reese,''' "[[Recap/PersonOfInterestS01E04 Cura te Ipsum]]"
->''"You can't keep playing God."''
-->--'''Carter,''' "[[Recap/PersonOfInterestS01E09 Get Carter]]"
->''"There are things you can do Detective, and things you can't. And that's where I come in."''
-->--'''Reese,''' "[[Recap/PersonOfInterestS01E21 Many Happy Returns]]"

!![[MyGreatestSecondChance Second Chances and Redemption]]
->''"I believe everybody deserves a second chance."''
-->--'''[[VictimOfTheWeek Andrea Gutierrez]],''' "[[Recap/PersonOfInterestS01E12 Legacy]]"

Finch offers Reese a second chance at life; Reese and Finch's interventions give many of the [=POIs=] a second chance; Fusco tries to take advantage of the opportunity for one which Reese unintentionally extends to him. In Fusco's case, his continuing association with Reese and Carter leads to his redemption, as he explains to Simmons in "[[Recap/PersonOfInterestS03E10 The Devil's Share]]":
->''"I could've been just like you, a bottom-feeder who turns on his own kind. For what? Money, power? I got lucky. I had a partner. She was good for me. For a lot of reasons. She reminded me that I could be good again too. I could be a good father, a good friend, a good cop. I'm not gonna let you undo all the good she did. Carter saved my life. She-- she saved me from myself...because she believed in me. And I'm not gonna throw all that away on a piece of crap like you."''

!![[TheAtoner Past Mistakes]]
Many of the characters are dealing with the consequences of, or atoning for, their past actions: Reese and Finch, of course; Fusco trying to escape his DirtyCop past; the [=POIs=] in "Mission Creep," "Triggerman," and "The High Road," to name a few. Summed up by Finch as he attempts to [[TalkingDownTheSuicidal talk a POI out of committing suicide]]:
->''"Your mistakes, like mine, are part of who you are now. You can't move on from that. Believe me. I've made a sizable number. But... sometimes your mistakes can surprise you. My biggest mistake, for instance... brought me here. At exactly this moment when you might need some help."''
-->--"[[Recap/PersonOfInterestS02E11 2-Pi-R]]"