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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.

  1. 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)
  2. 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)
  3. 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)
  4. 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.

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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:

Physical interaction with the image reveals it to be an illusion, because things can pass through it.

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:

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.

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.

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I think an important part of the spell text to consider when looking at your first two assumptions is:

A creature that uses its action to examine the image can determine that it is an illusion

(emphasis mine)

This leads me to believe that by RAW, your first two scenarios are not possible, because the spell specifies that one must either:

  1. Physically interact with the illusion, or
  2. Use their action to examine the image, and thus make an investigation check

Since one's passive Intelligence (investigation) would not use an action, you cannot passively determine the image is an illusion.

Your third and fourth scenarios are both correct interpretations of RAW, as far as I can tell.

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    \$\begingroup\$ Just to note, there are ramifications to that. You cannot throw an illusion over your allies and tell them it's an illusion, and have them automatically disbelieve (ex, create a false fog). Your enemies must also each make an investigation check to disbelieve your Silent Image/Minor Illusion of a still, unmoving fire (the most common example of an "obviously fake" illusion). Otherwise, they must treat the fire as real flame. \$\endgroup\$ – user27327 Mar 17 '16 at 11:14
  • \$\begingroup\$ @markovchain That's correct, it appears it's not possible to tell someone the image is an illusion and let them skip the Intelligence check. Every creature must 'see it for themselves', so to speak. \$\endgroup\$ – Shamwowters Mar 17 '16 at 11:18
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    \$\begingroup\$ @LinoFrankCiaralli Silent Image is visible to all, without a target, it's not an illusion in the vein of Phantasmal Force. Finally, you're right, the spell says nothing about unmoving illusions -- but I'm just showing a corner case with this spell, which shares issues with the Minor Illusion cantrip. \$\endgroup\$ – user27327 Mar 17 '16 at 11:30
  • \$\begingroup\$ "Use their action to make an investigation check to discover if it is an illusion" I don't think you need to investigate for that specifically. I think you just need to investigate it. \$\endgroup\$ – Mag Roader Mar 17 '16 at 15:11
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    \$\begingroup\$ Just as a note, this is also how they rule this spell in AL sanctioned play. Since the spell says nothing about passive perception, passive perception is void and each creature is simply assuming the illusion is real since they are busy focusing on other things, be it in combat or out of combat. Only a creature taking the time (using their action) to focus their perception to make an intelligence check against the illusion would have a chance of determining it's false. Otherwise physical interaction is the only other way. Very powerful when they can't see it coming. \$\endgroup\$ – Airatome Mar 17 '16 at 16:29

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