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I am specifically asking about the many spells (e.g. major image, silent image, etc.) that contain the following text or similar:

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.

The RAW answer seems to be no, even when considering outlandish illusory effects.

A creature can, perhaps, be certain of himself that what he is perceiving is an illusion. However, it seems that the illusion would not appear faint until interacted with or investigated. It does not seem to me that simply by observing an illusion and deducing it for what it is, no matter how sure of himself a creature may be, would constitute interacting with it or (especially important when in combat) investigating it with an action.

Vylix raises a good point in his comment: Being subjected living in a world full of magic, can a creature really know that "something is clearly impossible"? For instance, apple falls to the ground is a nature's law, but having an apple falls to the sky is not at all really impossible, provided there is magic involved: reverse gravity spell.

I've generated some confusion. To clarify:

I'm not asking about whether or not a creature can reach the conclusion that he is looking at an illusion and can react to it logically. A creature certainly can make that conclusion and reaction on his own.

More specifically, I'm asking whether or not the illusion can become faint to that creature simply because that creature is sure of himself that what he is seeing is an illusion. Or, must that creature observe a physical interaction with it, or spend his action investigating to achieve this?

This matters because being able to see through things is important, so is being able to hear things that are masked by illusory sounds, etc. Just because you are not fooled by an illusion, doesn't mean it isn't still interfering with your senses.


Possibly related: this answer to "Are Illusionists subject to their own illusions?".

This answerer suggests that a caster need not interact with or examine his own illusion in order to see through it. There are some ridiculous conclusions to draw here about an illusionist being susceptible in some fashion to his own spells. Perhaps casting an illusion is enough of an interaction so as to reveal it to the caster as fake.

Note that the accepted answer implies a caster may actually be susceptible to his own illusions. I digress.


Related: Does Minor Illusion break only when someone spends an action to investigate it?

...Though this question and its accepted answer do not fully address what I'm asking, only how much intent is necessary between an interaction and an illusion.

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  • \$\begingroup\$ How are you getting a RAW answer of "no" from the text you cited? \$\endgroup\$ – Mark Wells Jan 13 at 1:36
  • \$\begingroup\$ If spells do what they say they do, then either I've missed something or it's simply that those two methods (the action spent on investigating, or the interaction with the illusion) are the only ways an illusion can be discerned and made faint; there's no other written rule for discerning an illusion. \$\endgroup\$ – Token Jan 13 at 1:57
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    \$\begingroup\$ I'm not really sure what you're asking. The header made sense to me, but I lost the question in the ensuing text. Can you clarify the question? \$\endgroup\$ – lightcat Jan 13 at 6:01
  • \$\begingroup\$ There are two question here. The title one is "can a creature be sure of the illusory nature?" (the answer is most probably "yes", because a creature can be sure of anything, the rules do not govern this). The second one is "will the illusion become faint without spending an action nor physical interactions". This one is also unclear. Could you provide an example situation, where it matters? \$\endgroup\$ – enkryptor Jan 13 at 12:41
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    \$\begingroup\$ "simply because that creature is sure of himself that what he is seeing is an illusion" So your hypothetical Joe is just walking down the street and suddenly decides to believe that a piece of wall is an illusion? \$\endgroup\$ – Szega Jan 14 at 0:10
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There are no grades of "knowledge"; you either know or you don't

If a creature discerns the illusion for what it is, the creature can see through the image

This means that if the creature knows it is an illusion, it can see through it. There is nothing in the text that would indicate that the two presented methods are the only ways for it to know.

Also, there are only these two states: knowing and not knowing, and there is no scale of certainty involved. If something makes a creature doubt the reality of the object, it can Investigate, but until then is subject to the illusion. If something clearly impossible happens, the illusion is broken for anyone observing.

It does not seem to me that simply by observing an illusion and deducing it for what it is, ... constitute investigating it with an action.

If you couldn't tell by looking, you would need physical interaction. In that case there would be no room for Investigation checks at all. That check is exactly that: observing the illusion with attention to detail, looking for errors or discrepancies. Like shadows falling wrong, faint shimmering or the like. On a successful check the creature knows it is an illusion.

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    \$\begingroup\$ Being subjected living in a world full of magic, can a creature really know that "something is clearly impossible"? For instance, apple falls to the ground is a nature's law, but having an apple falls to the sky is not at all really impossible, provided there is magic involved: reverse gravity spell. \$\endgroup\$ – Vylix Jan 13 at 10:50
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    \$\begingroup\$ @Vylix Well, yes, but we have to draw a line somewhere, and we draw it where something is impossible to us. As I said, the game does not recognize the state of "being somewhat sure". \$\endgroup\$ – Szega Jan 13 at 10:54
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Nothing in the spell's description says this can't happen.

The spell describes two ways that you can discern the illusion, but it doesn't say "If one of those two things happens, the illusion becomes transparent." It says "If a creature discerns the illusion". This leaves open the possibility of discerning it some other way.

It's also significant that the line about physical interaction explains why touching the object reveals it to be an illusion. Plain rules text doesn't do this. Magic missile doesn't say "Each missile does d4 damage, because it's about the size of a dagger." The reason to explain why is to provide guidance for rulings. The reasoning we're shown is that, if you see the illusion do something physically impossible, then you know it's an illusion.

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Yes, if DM use passive checks

Use (passive) perception, followed by passive investigation to substitute active investigation needed in the spell's description.

If your passive Investigation is equal or higher than the spell DC, you automatically notice something amiss and deduce it's an illusion. If not, you are oblivious, or merely suspicious, but doesn't find what's wrong and must spend an action to do active Investigation check to find out whether it's an illusion or not.


PHB, pg. 175, section Passive Checks

A passive check is a special kind of ability check that doesn’t involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the GM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.

Perception check is needed to notice clues that hints an illusion, and investigation check to deduce that it is indeed an illusion. This investigation check is unnecessary if passive investigation score already discern it's an illusion.

This is similar to Crawford's tweet:

The DM decides whether to use passive Perception. If you do use it, a check is unnecessary if a passive score already noticed something #DnD

As a note, Observant feat grants +5 bonus to your passive Perception and Investigation check.

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    \$\begingroup\$ I question whether that's an appropriate use of passive Investigation, since it amounts to automatic success in many cases where rolling a check would have a chance of failure. \$\endgroup\$ – Mark Wells Jan 16 at 19:41
  • \$\begingroup\$ @MarkWells it's like saying having PP 20 is op because you automatically detect all traps and hidden enemies. You invested something (int scores, probably observant feat) and got something out of it: ability to passively detect illusion. And int "only" useful for wizards. \$\endgroup\$ – Vylix Jan 16 at 20:27
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    \$\begingroup\$ I don't care that it's "op"; I care that it's not how skill checks work. You don't get to roll a d20 and then pretend that anything less than a 10 is a 10. \$\endgroup\$ – Mark Wells Jan 16 at 20:36
  • \$\begingroup\$ @MarkWells the rules say that's how passive skill check work. It might be counterintuitive, but the DM is free to use it or not, and how to. It can even be made at -5 for disadvantage, if the DM deems you only "glance". Alternatively, combine it with PP. \$\endgroup\$ – Vylix Jan 16 at 20:48
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    \$\begingroup\$ Are there any examples of using a passive skill against a spell save DC? \$\endgroup\$ – Mark Wells Jan 16 at 22:08
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If there has been physical interaction with it, yes; if not, no

The illusion is actually there. It is an effect caused by magic and it does what it says it does. Even if you know it’s an illusion, say, because you cast it (you are not immune from your own illusions), it still does what it says it does.

For an analogy, there have been plenty of times when I know I’m looking at a mirage - I still can’t not see it.

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  • \$\begingroup\$ maybe "can't unsee it" is a better phrase for your last sentence? \$\endgroup\$ – Vylix Jan 14 at 1:09
  • \$\begingroup\$ Of note: "you are not immune from your own illusions" - Yes you are. You cast it, you know beyond any doubt that it is indeed an illusion. You don't need to make a check against your own DC to discern something that you cast is an illusion, you already know it is. \$\endgroup\$ – Lino Frank Ciaralli Jan 14 at 3:27
  • \$\begingroup\$ @LinoFrankCiaralli can I get the rule reference for that please? \$\endgroup\$ – Dale M Jan 14 at 4:32
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    \$\begingroup\$ It's right in the illusion spells. "If a creature discerns the illusion for what it is..." - common phrasing in every illusion spell. The caster discerns it quite readily due to the nature of being the originating point of the spell. If I put out my hand and utter the spellcasting necessary to conjure an illusory bridge, and a bridge appears exactly where I told it to, then I can readily discern that I cast the spell I was casting and therefore know it's mine. \$\endgroup\$ – Lino Frank Ciaralli Jan 14 at 5:19

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