Is passive Investigation essentially truesight against illusions?

I have been thinking about this for a few days (since taking the Observant feat specifically) and have now seen my logic backed up in this question.

Let's assume a character has 20 passive Investigation. A level 13 caster with 20 INT, has a DC of (8+5+5) = 18.

Does the character see through the illusion automatically?

I am prepared to consider that there is a range requirement on this, which would be answered by the linked question, so this question assumes the character is already within whatever range is required.

• Is there such a thing as "passive investigation"? There's clearly a passive perception stat, but investigation would seem to require some deliberate thought. – ktt4d Jul 17 '19 at 17:13
• @ktt4d I think you're right... Apparently in the Alpha version there was such a thing, and there's at least one reference to it in the rules that probably shouldn't be there (in the Observant feat), but it's not supposed to be "a thing". – Reginald Blue Jul 17 '19 at 17:25
• @ktt4d the observant feat specifically buffs it, so it has to exist, it just doesn't do anything really obviously like perception does – SeriousBri Jul 17 '19 at 18:16
• @ktt4d It appears on the DnD Beyond character sheets, Passive Insight, Investigation, and Perception. However, I think getting a Passive Investigation of 20 is VERY difficult, as I think it's basically 10 + your Int modifier (from memory, you might want to double check that) – Kalmino Jul 17 '19 at 18:30
• Also note that there are numerous examples where passive perception has a higher DC than active perception (this answer shares one example) -- nothing is preventing that from also occurring for investigation. Maybe 18 is the active DC, but the passive DC is 5 higher, at 23. Since the passive DC isn't defined anywhere, it's up to the DM; it could even be impossibly high, so that active investigation is the only option. (The logic behind +5 is that's the standard disadvantage penalty for passive checks.) – Phlarx Jul 17 '19 at 18:52

No

Consider this text from the Major Image spell:

Physical interaction with the image reveals it to be an illusion, because things can pass through it. A creature that uses its action to examine the image can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC. If a creature discerns the illusion for what it is, the creature can see through the image, and its other sensory qualities become faint to the creature.

Passive investigation doesn't use your action so it doesn't automatically identify the illusion as being such.

• Damn, you ninja'd me x.x +1 – Jihelu Jul 17 '19 at 8:29
• Since it's very hard to get passive Investigation that high, I'd probably give the player a hint that something is strange about "that wall" (or whatever the illusion is) with his passive check - and then he can take an action with an active check to discern the illusion as per rules. – Falco Jul 18 '19 at 8:30

Probably not, no

As always, your DM can choose whatever he wants for this and may pick and choose per scenario, but let's look at the text of Minor Illusion.

If a creature uses its action to examine the sound or image, the creature can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC. If a creature discerns the illusion for what it is, the illusion becomes faint to the creature.

Passive Investigation is not the same as actively making a check, so it doesn't really qualify for the 'using your action' requirement. A DM may disagree with this line of thinking but it seems to be intended.

• Checks can be passive or active, IIRC – BlueMoon93 Jul 18 '19 at 7:11

Not quite but it certainly helps against them!

To give an example (as seen in Allan Mills' answer), see the description of the major image spell:

Physical interaction with the image reveals it to be an illusion, because things can pass through it. A creature that uses its action to examine the image can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC. If a creature discerns the illusion for what it is, the creature can see through the image, and its other sensory qualities become faint to the creature.

You still need to use your action to examine it, but passive Investigation can come into play by setting the floor for the roll. (Note, not all DMs will go with this, but I haven't seen a DM not go with it.) For example, as you said in your question:

Let's assume a character has 20 passive Investigation. A level 13 caster with 20 INT, has a DC of (8+5+5) = 18.

If the DC is 18 and your character has a 20 passive Investigation (for the sake of this example, I'll assume that your character is also a level 13 with a Intelligence score of 20 and is proficient in Investigation), then if they roll for the Investigation check and the roll before modifiers is a 7 or lower (+5 for Int mod and another +5 for proficiency for a +10 total), that roll totals 17, which is lower than the character's passive Investigation. In cases like this, you would use passive Investigation as the floor for the check, bringing the total roll up to 20. So your passive Investigation makes you, in this case, auto-pass any check you make with a DC of 20 or lower.

• – Himitsu_no_Yami Jul 18 '19 at 20:18
• One of my DM's plays this way, although the others haven't ran into such a situation yet. I would certainly rule this way if I was a DM, but am conscious that it isn't part of the rules and I do understand the arguments against it. – SeriousBri Jul 19 '19 at 6:46