Perception or Investigation?

Hey ho. Another D&D 5e post here. If you're not into D&D you can skip it. Eventually I might compile a bunch of D&D-related posts into a separate blog.

I want to search for traps.

Do I notice the orc ambush before it is sprung?

Do I see the incriminating letter that has slipped under the Burgomeister's bed?

I want to search in a wizard's library for any books that might be able to explain the weird necromantic events that have been plaguing the town of Littlebridge.

Do I roll Wisdom (Perception), or Intelligence (Investigate)?

This is the stuff that table arguments are made of. The final answer of course is that the DM decides what you get to roll. However, I hope to provide a bit of guidance on this question in order to forestall such arguments in the future.

The basic difference is whether you are actively doing something in order to gather information about a situation or a thing, or whether you notice it using only your senses. The former is Investigation, and the latter is Perception.

Perception checks can be Passive, which essentially means that you don't roll the check - you compare your Passive Perception value against the difficulty of noticing the thing. Investigation checks generally aren't Passive - I'm not even sure what a Passive Investigation check could possibly mean. The DM has to decide whether to use Passive Perception or to get the player to roll. Generally if there are no immediate consequences to failing the check, it is better to use Passive Perception. Otherwise the DM risks putting the players on guard for something that their characters are unaware of.

It's important to remember that a skill check is actually an attribute check, with your Proficiency Bonus added if appropriate. Intelligence is used to make checks that relate to mental effort. Wisdom checks are used for situations where intuition or common sense is involved. A perfectly valid rules variant would be to not use skill proficiencies at all. Instead, when the player says what action they undertake, the DM tells them to roll one of the six attributes, and the proficiency bonus is added if it seems appropriate. Skill proficiencies are intended to make this process easier, but there's absolutely nothing in the rules to say that the DM can't call for a Wisdom (Investigate) check or an Intelligence (Perception) check, or some other variant.

Here's an example of that. A Wizard is giving a presentation to a group of other Wizards on some new aspect of the arcane that they have discovered. The DM called for a Charisma (Arcana) check to see if they are convinced.

For the remainder of this article, however, we will refer to the standard skill proficiencies - Intelligence (Investigate) and Wisdom (Perception).

So let's look at our four examples above. Searching for traps is, in most cases, Investigate. You are actively looking for something - you are testing, manipulating, and yes, observing, to learn whether the door or chest or hallway contains something that is going to hurt you. Contrast this with opening a door or walking down a hallway without checking first. If the trap has a visible component, the DM may ask you to make a Perception check to see if you notice that tripwire before you stumble into it. In either case, stumbling into the trap without having noticed it will trigger a Saving Throw to avoid the consequences of having set off the trap.

The orc ambush is a Perception check, opposed by the orcs' group Stealth check. This is different from playing hide-and-seek with a child, because in a game of hide-and-seek, you are actively looking for the child - opening doors, looking under beds, shifting curtains. That would be Investigation unless their foot is sticking out of their hiding place - in other words, something that you might be able to notice just by walking into the room.

Finding the Burgomeister's letter could be either Perception or Investigation, depending on what the party says they want to do. "I look around the room - do I see anything interesting?" is Perception. They're not doing anything to find something, they're just standing in one place and looking. "I search the room for anything interesting or unusual" is Investigation. In this case, it is assumed that they are looking under things, opening drawers, moving stuff and actively hunting for something.

Searching the wizard's library is definitely Investigation. A specific passage in a specific book is not something that someone might be able to notice just by walking into the room - you would have to be actively looking for it. You would need to work out how the books are shelved, pick out a book from the correct section, locate an appropriate chapter, find the passage that's relevant to what you're looking for.

I hope I've clarified the distinction here. In a nutshell, if it's something that it might be possible to notice with just your senses, use Wisdom (Perception). If it is something you have to actively hunt for, use Intelligence (Investigate).