I love illusions, including Silent Image, but unfortunately I don't think it's always handled correctly. I'd like to clarify the RAW interpretation of this spell for this purpose, specifically when believing or disbelieving the illusion.
Here is the text of the spell:
Silent Image,
1st-level illusion[..] 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.
Four Scenarios
There are two axes relevant to this question. Active vs Passive Investigation and In vs Out of Combat. I divided them this way because they seem to me to be the two most significant factors when investigating an illusion.
Active vs Passive have different mechanics by RAW, whereas In vs Out Of Combat is the difference between the illusion being actively used against you or simply finding the illusion already in place after it's been cast.
-
Passive, in combat
- When a character conjures an illusion of a hostile creature out of your view, as with Silent Image, and uses their action to make it move realistically, but out of your reach.
- This would presumably use Passive Intelligence (Investigation)
-
Passive, out of combat
- When you stumble into an illusion of a bridge over a 15-foot chasm (the chasm is real), with the illusionist nowhere in sight, and you need to cross it, but having no reason to disbelieve the bridge.
- This would presumably use Passive Intelligence (Investigation)
-
Active, in combat
- When you see the caster create an illusion of a fog cloud to obscure your vision of the caster's allies, but out of your reach.
- This would presumably use normal Intelligence (Investigation)
-
Active, out of combat
- When you see three doors as part of a puzzle, are notified two are illusions, and you must choose the real door on your first try.
- This would presumably use normal Intelligence (Investigation)
Assumptions
Here are my assumptions, please correct me if I am wrong/not in line with RAW/RAI:
- If a creature touches the illusion, they automatically discern the illusion and can see through it.
- Any investigation checks that require a roll cannot involve touch, as an investigation involving touching the illusion cannot fail, therefore not requiring a roll.
- If the investigation does involve touch, there is no roll. It is automatically a success.
Having laid out my premise: how does one investigate (with dice rolls, so as not to auto-succeed) a Silent Image? If possible, it would be great if the answers address the four specific scenarios I've listed, and validate/refute my assumptions.
Best Answer
Addressing your concerns in order:
Spell located in PHB pg.276
Passive, in combat
The only way to check it in combat passively is to pass something through it. Silent Image does not resist this as the wording of the spell states:
However, in combat a creature doesn't naturally assume something is an illusion and attempt to wave a hand through it or run into it's space. This is why it requires an active check, which will be covered later.
Passive, out of combat
If you try to step on an apparently real bridge and tumble through, you would passively know that it was an illusion. This is covered much the same as above, in combat.
Active, in combat
This is the way the spell is actually worded. Specifically:
This means that they have to use active checks in order to discern the image is an illusion if they're in combat. The reasoning behind this would be simple logic. If you were engaged in combat and an Earth Elemental suddenly rose from the ground, how certain are you that it's an illusion and not a spell? What are the consequences for being completely wrong if you tried to just ignore it and move into it's space? An active check is based on somebody thinking it's not real, and then testing it to make sure, but testing it relatively safely.
Consider that testing it may include prodding it with a sword or tossing a rock through it to check to see if it has substance. Basically, something that requires an action to attack, follow through, and wait to see the result. It's not a bad use of an action as it effectively defeats a hazard in a mere 6 seconds (so long as you pass the DC check!)
Active, out of combat
These are your typical take 20 scenarios. Any active check out of combat is something players can simply repeat over and over to ensure that what they're looking at is very likely to be real. However since the spell states that physical interaction reveals the illusion for Silent Image, the basics of an active investigation should be touching it with something, which would reveal it's nature.
Lastly:
You are correct: Any physical interaction sees through Silent Image. Be careful if utilizing this knowledge in game (meta-gaming) because as a DM I would start mixing in invisible creatures suddenly popping up and giving them opportunity attacks if you try to casually interact with them physically in the middle of combat. Reaching out to see if that Drow Warrior is really real is a great way to lose a hand or arm.