Part of an ongoing series of 5e Rules notes.

Perception and Investigation

Once upon a time D&D had things like “Detect Traps” and “Disarm Traps” and lots of other very specialized skills for spotting and understanding and doing things about hidden things, and dangerous things, and dangerous and hidden things.

5e’s mechanics are arguably simpler and cleaner and more straightforward … but understanding when you should use them is definitely not. Questions always come up …

Is this a Perception roll or an Investigation roll? Or should this be done as Passive Perception?

If a module says one way or the other (“The trap can be found with a Perception roll of 13 or better”), that’s generally the way to do it. Most DMs, in lieu of that, default to their favorite.

Intelligence and Investigation

Intelligence measures mental acuity, accuracy of recall, and the ability to reason. Intelligence checks are used to draw on logic, deductive reasoning, memory, knowledge, and/or education.

Investigation: The ability to put together clues and make deductions the others wouldn’t make. E.g. …

“There should be a hidden panel right there.”
“This wound was caused by a stiletto.”
“He’d want his magic wand close at hand in case his pursuers found him. I’m going to look in the bedding and side table.”
“Are any of the books in this book case levers for a secret door?”
“That’s the point where the tunnel is most likely to collapse.”
“It took me a while, but I found where this scroll refers to an item in our quest.”
“What do I think this room was used for?”
“Good spotting those scratches on the floor. Here’s what I think they mean.”
“I go through the files to see if they tell us who the wizard’s father is.”
“That patch of floor you pointed out … it looks like if you step there, it will depress and, I suspect, trigger a trap.”
“How large of a creature made these tracks?”
“I’ve never seen a trap like that before. Let’s figure out how you can disarm it.”
“The baron is incredibly vain. I’m going to focus my search for the map behind his giant portrait.”
“Can I figure out why the shopkeeper is so angry?” (That he is angry is, if needed, an Insight roll.)

Just trying to find something is not an Investigation; it’s Perception. Using what is obvious (or has been Perceived by you or someone else) to figure something out is Investigation.

Wisdom and Perception

Wisdom reflects how attuned you are to the world around you and represents perceptiveness and intuition. Wisdom checks are used to test that.

Perception: This is used to spot, hear, or detect something’s presence, usually using natural senses, esp. if they are obscured or easily missed. It represents general awareness of surroundings and keenness of senses. Perception is the player’s ability to spot and detect people or items; it has nothing to do with making intelligent decisions about those people or items.

“I can hear them talking on the other side of that door.”
“I’m looking for tracks.”
“Something is creeping forward through that brush.”
“Is there anything strange about that crowd of people?”
“I smell something disgusting.”
“I am looking for a secret door.”
“I think I spotted folk lying in wait up there.”
“I heard footsteps following behind us–but I don’t see anything.”
“The floor tiles in that part of the room are a little different color.”
“Ah, there’s the sign for the street he told us to turn on.”
“Wait, there’s light coming from under this stone wall; there might be something behind it.”
“I need more arrows; I’m keeping my eyes open for an armorer.”
“I’m searching his quarters for any paper or documents.”
“Ah, there are some dandelions I can use for that potion.”
“At this point, I think this tunnel is passing right under the castle.”
“There’s a needle trap on this chest; I’ve seen this kind of thing before.”
“Is there anything unusual about this room?”

Are you attempting to hear, spot, or detect something? Are you using your senses (sight, smell, taste, touch)? Does what you’re trying rely on awareness of the surroundings (or one focused point)? Are you attempting to merely perceive or find something, not determine a deeper, hidden message? Then it’s Perception.

Investigation and Perception

Just because you can Perceive a clue, doesn’t mean you can understand it; that’s where Investigation comes in. Perception might notice small holes in the wall. Investigation would determine they are a dart trap — or perhaps vents allowing a door to close, or recessed buttons to open a secret panel.

The DMG, p 238, also notes:

If you have trouble deciding whether to call for an Intelligence or a Wisdom check to determine whether a character notices something, think of it in terms of what a very high or low score in those two abilities might mean.

A character with a high Wisdom but low Intelligence is aware of the surroundings but is bad at interpreting what things mean. The character might spot that one section of a wall is clean and dusty compared to the others, but he or she wouldn’t necessarily make the deduction that a secret door is there.

In contrast, a character with high Intelligence and low Wisdom is probably oblivious but clever. The character might not spot the clean section of wall but, if asked about it, could immediately deduce why it’s clean.

Wisdom checks allow characters to perceive what is around them (the wall is clean here), while Intelligence checks answer why things are that way (there’s probably a secret door).

Ideally, one would have someone with high Perception to find things, and then someone with high Investigation* to understand what those things mean.

^{*Or other INT-based Ability.}

Another point to consider: the underlying question from the GM should always be, “What is your character doing?” “I’m searching the chamber” is perhaps too vague — are you going along and tapping all the walls? Are you standing in the middle and getting a sense of the layout of things? Are you lifting up each crate, or rifling through each drawer? What are you searching for, and how are you trying to find it? That can make clear if you are Investigating or Perceiving (and affect what a successful roll means).

And all that said, it’s important not to make things too cumbersome. “All right, Bob, make a Perception roll to see if there’s something off about that chest lock. Okay, Susan, you’ll need to Investigate to identify the trap that Bob is found. Excellent, now Ted, make a Dexterity (Thieves Tools) roll to disarm it.” That’s an accurate way to do things, but also kind of clumsy. Most cases described in official materials related to traps tend to go Perception+disarm rolls, leaving out the Investigation piece. That makes sense for normal, “obvious” traps. For more elaborate traps, that may not be the case.

(Another idea I’ve seen is letting an added Investigation check be made to gain an Advantage on the disarm.)

And a final thought: The question is always, “How can we best have fun?” The rules and interpretation and all should be promoting that end, not defeating it. Depending on how we see what “fun” is (challenge, competition, story, humor, simulation, etc.) may create variation, but worrying too much about “Are we following every rule to the letter, and where the rules are ambitious how can we succeed in our duty?” is not (except for the extremely LN amongst us) most likely to get us there.

References

Here are some places to look for more information, including some good web pages for reference (much of the material there has soaked into the above):

Bonus Topic: Passive Perception (Again, Still, Some More)

That there are Active and Passive versions of most skills is clear in the rules. I still find it frustrating in how Active vs Passive Perception is described and its varied applications (let’s hope the new 2024 not-an-edition of D&D gives us a bit more clarity on this)

Then this SkullSplitterDice article caught my eye, and I found it summarizes a lot of the debates and struggles I had over the concept of Passive Perception and what it means and when one can or should use it, so I’m going to walk through it and comment.

Passive Skill Checks are used, the article suggests, for three reasons:

1. “To gloss over a lot of time attempting the same thing over and over.”

Or, as it says as an example on PHB 175, “Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again.” This is roughly like “Take 10” in D&D 3.5. If there’s no cost to re-attempting (re-rolling) or failing (or consequence to the time it takes), then the Passive Skill lets the GM simply say, “Yeah, you eventually do it” if it is higher than the DC of the obstacle.

From a Passive Perception standpoint, this would be difference between, “I am going to search the room” and “I am going to spend an hour searching and re-searching the room, because that secret door has got to be here somewhere.” The former would be an Active roll; the latter a Passive check.

The problem is, if the DC is 20 and your Passive Perception is 15, you can theoretically spend forever doing it, Passively, and never find that Secret Door.

Plus, this stops mattering with the rule on DMG 237 for “Multiple Ability Checks”:

Sometimes a character fails an ability check and wants to try again. In some cases, a character is free to do so; the only real cost is the time it takes. With enough attempts and enough time, a character should eventually succeed at the task. To speed things up, assume that a character spending ten times the normal amount of time needed to complete a task automatically succeeds at that task. However, no amount of repeating the check allows a character to turn an impossible task into a successful one.

That takes a lot out of “re-rolling,” rendering this aspect of Passive Skills moot.

2. “To average out a lot of minor consequences.”

The example given here is using a Bard’s Perform as a Passive to represent how they do on any given night, rather than rolling for each hour, or each bar, or each song. (One could question, in this case, whether the consequences of a spectacular performance before the right crowd, or a complete botch-up of one before the wrong crowd, might not be so minor.)

I’m not sure how to fit that into Passive Perception, however.

3. “To allow the DM to get check results without the players knowing about it.”

This always comes up, as if to trump any and all other vagueness. “Use this, because then you don’t have to warn the players that something is up by having them roll or by your rolling behind the screen.” Very meta.

Also, very useless if the GM is using macros in a VTT like Roll20. One click and I can generate the party’s Active Perception rolls, as easily as having their Passive Perceptions on a piece of paper in front of me. (In fact, the macro I use shows the Passive and a thrown Active Perception for each player, so it’s literally the same effort for me.)

So this use of Passive Perception doesn’t gain anything.

All that said …

I will add two more uses I read about that aren’t included in the article:

4. To provide a basement for Active Rolls.

This is a weird one, and while used by a lot of people (including some important WotC folk), it bugs me. The idea is that if Passive Perception is, say, 12 … you can never roll on Active Perception below a 12, because, well, if Passive Perception is what you just automatically notice, how could you Actively notice any less than that?

I disagree, for two reasons:

First, Passive Perception in this case is normal conscious and unconscious awareness in a situation.

If I start peering at the bookcase looking for a crack that signals a secret door, I should very much be more likely to find it than if I just glanced around the room — but I am also less likely to notice the crack in the opposite wall.
If I’m worried there are Orcs in the bushes ahead, I might be distracted from noticing the Roper in the tree above me.

I.e., my Active Perception can definitely “roll” lower than my Passive Perception.

Second, the Rogue’s Reliable Talent ability at 11th Level literally does this basement thing.

By 11th level, you have refined your chosen skills until they approach perfection. Whenever you make an ability check that lets you add your proficiency bonus, you can treat a d20 roll of 9 or lower as a 10.

That’s essentially the level of a Passive check. If we treat Passives as the basement for an Active roll, this Class Feature is meaningless. Boo.

5. To let monsters search for you without a die roll.

There is one other use case the article doesn’t mention, and, interestingly, it’s the only place in the rules that really spells out a case of using Passive Perception:

When you hide, there’s a chance someone will notice you even if they aren’t searching. To determine whether such a creature notices you, the DM compares your Dexterity (Stealth) check with that creature’s passive Wisdom (Perception) score, which equals 10 + the creature’s Wisdom modifier, as well as any other bonuses or penalties. If the creature has Advantage, add 5. For Disadvantage, subtract 5. For example, if a 1st-level character (with a Proficiency Bonus of +2) has a Wisdom of 15 (a +2 modifier) and proficiency in Perception, he or she has a passive Wisdom (Perception) of 14.

(In general skill contests between players and opponents, 5e leans toward die rolling on the player side (for obvious feelings of having agency). While some contests are roll-vs-roll, if there is one side that will not be rolling, it will be the non-player side.)

The question is, why use the the non-PC’s Passive Perception? Why not have the monster roll, too? It’s literally just as easy for the DM to do that.

So what does all this mean?

There are a lot of folk who lean heavily on using Passive skill rolls, including for Perception.

But all the above means I’m not likely as a DM to do much with Passive Perception or other Passives. They don’t add much other than letting the DM be sneaky, and I can be imperceptibly (see what I did there?) sneaky without them.

The only thing that might change that would be if someone in my game took the Observant feat, which adds +5 to your Passive Perception and Passive Investigation. That might make me re-evaluate all this, which I’d rather not, so I hope they don’t.

Bonus Topic: Other Intelligence checks

Intelligence (Investigation) tends to be about the here and now, figuring out present clues and circumstances to deduce and understand them. Other Intelligence abilities are more academic.

Use Intelligence (History) to recall lore about historical events, legendary people, ancient kingdoms, wars, locations, etc.

Use Intelligence (Arcana) to recall lore about spells, magic items, eldritch symbols, magic traditions, planes of existence (and their inhabitants).

Use Intelligence (Religion) to recall lore about the gods, rites and prayers, churches, holy symbols, and cults.

These three are largely not interchangeable, at least in terms of the info they give, though there could be some overlap. For example, imagine standing in front of a statue of a woman in regal robes and holding a staff with an interesting design on it.

History would potentially inform you of who that is, or maybe least their era or realm (based on the fashion style). The style of the sculpture might also provide some further information; if the statue itself is or resembles a famous one, the lore behind it might be revealed. The crown on her head, the pattern on her cloak, a national symbol on a necklace could provide clues.
Arcana could tell you about that staff and its history and abilities. Or it might recognize who the figure is if they were a ground-breaking spell-caster or researcher. Their gloves might have warding symbols against fire, indicating who their opponents were. If they are in the midst of spell-casting (showing a somatic element), that might be identifiable as well.
Religion might inform you about that holy symbol woven into their robes, or if this matches a recorded form of a deity. It might also tell you if their stance or garb matches a particular faith practice or ritual, with possible further info (including time and place) stemming from that.

Also, of course, use Intelligence (Nature) to recall lore about terrain, plants, animals, weather, and natural cycles. That might not help with the statue … except it could assist with recognizing the stone and where it’s from, or how the wear pattern indicates an age, or even, if creatures or plants are depicted in the statue (the flowers at her feet, the dragonet sitting on her shoulder) provide information about the time or place being referenced.

D&D 5e Rules – Perception and Investigation (and Passive Perception)!