The RAI of Illusions and Physical Interactions

[Skylivedk]

Hello Playground,

So I have a question. The RAW is quite clear in this text (example from Silent Image):

"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."

But, how would you rule the illusion of something immaterial: fog, poison cloud, misty vampire, ghosts, etc?

[Unoriginal]

Hello Playground,

So I have a question. The RAW is quite clear in this text (example from Silent Image):

"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."

But, how would you rule the illusion of something immaterial: fog, poison cloud, misty vampire, ghosts, etc?

Those things can still be perceived by touch (fog is wet, ghosts are cold, etc). Illusions like Silent Image don't give any sensory stimuli when you have a physical interaction with them, revealing them to be illusions.

[DrowPiratRobrts]

Agreed with Unoriginal. Though the RAW are actually increasingly unclear the further down the rabbit hole you go. Some say that once you know something is an illusion you still have to pass a check to see through it. Some say you don't need the check if you physically interact with it. Then there are different rulings for what qualifies as physical interactions and whether or not you have to be the one interacting with the illusion. It's actually quite complicated when people are committed to taking every possible side of the RAW. English is hard. RAAAAAWRRRRRR!

[Chronos]

Physical interaction doesn't necessarily mean touching something, though. If I see what looks like a person but my sword passes right through it, well, maybe it's an illusion or maybe it's a ghost, and it didn't feel cold because I didn't touch it, my sword did. Nor is a ghost that still looks like a solid person much of a stretch, for a D&D-style fantasy world (there's probably some sort of incorporeal undead already in the books that looks like a solid person).

[Mjolnirbear]

To my knowledge there's no RAW for illusion interaction in general, only for each spell, and they're wildly different.

Creation makes items out of illusion. Your object's transience depends on what you have made it out of. There is no disbelieving Creation.

Phantasmal Force's interactions result in damage where appropriate and are rationalized away by the subject.

In general, if a spell allows disbelief, I follow the rules of the spell. Where "physical interaction reveals it" I apply some logic: who or what did the interaction? Who witnessed it? What limitations are on the illusion? A creature or arrow passing through fog makes logical sense, unless the creature is not an ally, because fog is cold and damp. I walk into the fog, no disbelief. Orc Thug 3 walks into the fog after me, he notices a lack of feeling. I walk through an illusionary wall, however, and everyone disbelieves. While there are spells that let you walk through physical objects, KISS says it's more likely an illusion unless there is reason to believe otherwise, such as something else hitting the wall normally.

[Skylivedk]

Those things can still be perceived by touch (fog is wet, ghosts are cold, etc). Illusions like Silent Image don't give any sensory stimuli when you have a physical interaction with them, revealing them to be illusions.

I'm mostly concerned about arrows and the like. Full acceptance of anything you can feel to be different revelator that it is different. Arrow through ghost though - RAW you know it's an illusion.

[Chronos]

Arrow through ghost though - RAW you know it's an illusion.
Even if it's not?

[druid91]

I wouldn't call shooting an arrow at an illusion physical interaction.

[Skylivedk]

I wouldn't call shooting an arrow at an illusion physical interaction.

Curious, why not?

[Mjolnirbear]

Curious, why not?

Don't know about Druid, but it makes more mental sense to me that 'touching a wall and not feeling a wall' is what reveals the illusion, not 'matter disrupts the illusion.'

If your different senses are contradicting each other then you learn of the illusions exostance and disbelieve it.

If it was absolute physical interaction, then illusions would be broken up by gusts of winds or you could blow on it to reveal it. It wouldn't be a great illusion if your average draft caused it to flutter and die.

[ad_hoc]

Don't know about Druid, but it makes more mental sense to me that 'touching a wall and not feeling a wall' is what reveals the illusion, not 'matter disrupts the illusion.'

If your different senses are contradicting each other then you learn of the illusions exostance and disbelieve it.

If it was absolute physical interaction, then illusions would be broken up by gusts of winds or you could blow on it to reveal it. It wouldn't be a great illusion if your average draft caused it to flutter and die.

Only if you should be able to see the effect of the draft. A Gust of Wind spell targeting an illusory flag should be able to reveal it as an illusion for example.

An arrow is a physical thing that is interacting physically with the illusion. That's the definition of it.

Physical interaction doesn't cause illusions to 'flutter and die', they just allow the observer to know them as illusions.

[Tanarii]

The RAW is not clear. There are several things a DM needs to rule on:

1) does physical interaction make the illusion go faint, or is that just a clause of the Investigation check?

2) Does physical interaction require perception of the interaction?
2a) If yes, does physical interaction work for anyone that percieves it, or only the person interacting?
2b) Can the perception of interaction happen at a distance?
2c) How do you determine who may perceive a specific interaction, especially at range?

And of course, the one you came up with:
3) what counts as physical interaction for something that naturally allows things to pass through, like a visual phenomenon (fog, mist, rain)?
3a) Is anything just being in the area of the illusionary phenomena (fog, mist, rain) automatically perceived as an interaction, provided perception is required and it can be perceived?

Edit: FWIW, I rule:
1) The going faint rule is a subclause of making an investigation check. Physical interaction just lets creatures know it is an illusion and act accordingly.
2) physical interaction is seen by anyone who sees it. If there is a question about that, that's what perception is for.
3) phenomena (rain, mist, fog) interact with everything in their area in a way that reveals that it is an illusion, but you must be able to percieve the interaction. Otoh because of #1, it still incredibly useful since it continues to be opaque, until any given creature spends an action to investigate the illusion.

[ad_hoc]

The RAW is not clear. There are several things a DM needs to rule on:

1) does physical interaction make the illusion go faint, or is that just a clause of the Investigation check?


I think where people go wrong with illusions is that they try to use them mid-combat. Then they say, hey, it's too easy to physically interact with them so that means they're bad.

Yeah, they're bad in combat. And that's fine. There are good enchantment and illusion spells for combat. Minor Illusion/Silent Image/Major Illusion aren't them.

Illusions set up ahead of time have a lot of potential.

[Merudo]

Some DMs just have an irrational dislike for illusions. You'll have folks here claim loud and clear that a single arrow can totally expose "Invoke Duplicity" for what is it, even though the ability description says it creates "a perfect illusion of yourself".

Apparently a "perfect illusion" is quite flawed for these folks.

[ad_hoc]

Some DMs just have an irrational dislike for illusions. You'll have folks here claim loud and clear that a single arrow can totally expose "Invoke Duplicity" for what is it, even though the ability description says it creates "a perfect illusion of yourself".

Apparently a "perfect illusion" is quite flawed for these folks.

Well the creatures will certainly know something is wrong. That ability doesn't have the text that says it becomes faint upon interaction so it remains.

It is likely that sentient creatures would try something else than continuing to fire at that illusion such as attacking other party members, running away, using magic, searching for other foes/the source, etc.

[Mjolnirbear]

Only if you should be able to see the effect of the draft. A Gust of Wind spell targeting an illusory flag should be able to reveal it as an illusion for example.

An arrow is a physical thing that is interacting physically with the illusion. That's the definition of it.

Physical interaction doesn't cause illusions to 'flutter and die', they just allow the observer to know them as illusions.

That's what I mean by senses contradicting each other. A flag not flapping when you feel a breeze = illusion. Arrow that passes through a dust cloud illusion? Nothing in that that screams something is wrong. Arrow passing through solid brick with no sound? Illusion. Witnessing the physical interaction is what triggers it.

The other interpretation, the one I don't run, says basically illusions are degraded by matter. Even with no one there to see it, if a blind mole walks or falling leaf falls through the illusary brick wall, then the illusion is degraded even if it takes three years before someone happens upon the illusion. Like the magic is disrupted by the passing of matter.

I wasn't certain I'd explained myself adequately so I wanted to make this clear.

[Agent-KI7KO]

Have we come to a ruling? I myself use minor illusion as a wall to prevent enemies from seeing what I’m doing - does the wall become faint upon an arrow passing through it?

[Ganymede]

If I see what looks like a person but my sword passes right through it, well, maybe it's an illusion or maybe it's a ghost, and it didn't feel cold because I didn't touch it, my sword did. Nor is a ghost that still looks like a solid person much of a stretch, for a D&D-style fantasy world (there's probably some sort of incorporeal undead already in the books that looks like a solid person).

This is simply not convincing. Illusions do not react when you slice at them with a sword. To contrast, ghosts definitely react. It does not matter if you can't feel a ghost's chill through a sword (though that's debatable) because you can still readily identify the other ways in which a ghost reacts: its physical form wavers, there is a resistance on the blade, the ghost cringes from the inflicted damage and reacts with hostility. It is even possible to see the ectoplasmic residue of the ghost on your sword, but that's getting a bit more in how DMs describe ghosts.

It is the same with mist: even if you cannot feel the wetness or chill of mist, it still swirls and spreads about when disturbed. Illusions, at least lower level illusions, do not.

[Tanarii]

I think where people go wrong with illusions is that they try to use them mid-combat. Then they say, hey, it's too easy to physically interact with them so that means they're bad.

Yeah, they're bad in combat. And that's fine. There are good enchantment and illusion spells for combat. Minor Illusion/Silent Image/Major Illusion aren't them.

Illusions set up ahead of time have a lot of potential.They're a lot more effective if they don't go faint when physically interacted with. IMO and on reflection of many folks responses in previous threads, the interpretation that the physical interaction does cause them to go faint is probably a sounder strict reading of the sentences in question. But IMX it's far more problematic in play. Requiring a Investigation check to make it go faint allows them to retain decent power, while providing a better answer for DMs and Players in regards to a numerous set of questions.

(Note that I've found if you do this, even the caster should require an action to make his own illusion go faint. That prevents some shenanigans.)

[DrowPiratRobrts]

I think where people go wrong with illusions is that they try to use them mid-combat. Then they say, hey, it's too easy to physically interact with them so that means they're bad.

Yeah, they're bad in combat. And that's fine. There are good enchantment and illusion spells for combat. Minor Illusion/Silent Image/Major Illusion aren't them.

Illusions set up ahead of time have a lot of potential.

I disagree strongly that these spells you've listed aren't good for combat. I'm altogether shocked that you included Major Image in the list since it can affect sight, smell, and hearing and seems completely real. In addition you can make it move naturally. You may not get more than 2-3 rounds out of it, but that's pretty effective depending on what you need to use it for (distraction, retreat, etc.). Minor Illusion can easily be used to grant full cover. Just cast an image of a boulder over the Dwarf that needs to heal up a bit. If you argue that the Dwarf is "interacting" with the boulder, just make it hollow. RAW there's no reason you can't do this, and a good DM in most situations won't have their monsters attack a boulder that just appeared...you just have to be creative with illusions, but they are still good regardless of how DMs adjudicate the illusion rules, so long as the DM doesn't metagame.

For example, when I was DMing a while back the party was getting beat up by some Kuo-Toa in a swamp. Someone cast Major Image to make a giant snake appear as if it was coming out of the water and it started to make its way toward them. Kuo-Toa have a great fear of snakes, so I decided there's no reason any sensible Kuo-Toa would attack and I just made them run away as long as the snake pursued. IMO I think that's how DMs should approach illusions, rather than thinking that the first response of every NPC is to interact with the illusion or make an intelligence check right away. They may not have a reason to interact with the illusion physically or make an Int check to see if something is off, and I really believe that the DM needs a good in-game reason to do these things that test the illusion. I think a lot of DMs fall into this metagaming trap all the time, and that's part of why so many people think illusions are bad.


Have we come to a ruling? I myself use minor illusion as a wall to prevent enemies from seeing what I’m doing - does the wall become faint upon an arrow passing through it?

I would personally rule that it does reveal the wall, but I understand why some people don't. My question for the DM would be, why is someone firing an arrow at the wall? That doesn't make sense. I'd be upset as a player, and I try my hardest not to metagame like that as a DM. It's not fair because you always have all the info of your players actions, whereas they seldom have half the information about your own actions. Good DMs respect that rather than abusing it.

[Mjolnirbear]

Have we come to a ruling? I myself use minor illusion as a wall to prevent enemies from seeing what I’m doing - does the wall become faint upon an arrow passing through it?

RAW is vague an unclear. As in all such cases, "ask your DM" is the rule. Not in the least because individual circumstances, which may be different than OP's, may change things.

Because no DM when such a scenario arises has time mid-battle to say "let me see what strangers on internet forums say about this". DMs need to make quick rulings and get on with the game.

[Man_Over_Game]

RAW is vague an unclear. As in all such cases, "ask your DM" is the rule. Not in the least because individual circumstances, which may be different than OP's, may change things.

Because no DM when such a scenario arises has time mid-battle to say "let me see what strangers on internet forums say about this". DMs need to make quick rulings and get on with the game.

I'm not really sure what's vague or unclear about Minor Illusion and physical interactions:


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

Physical interaction = Investigation Check. They're just two different ways of reaching the same goal.

[Skylivedk]

I'm mostly curious about the edge case of arrows and rocks, especially in cases where they could pass through (fake water fall, incorporeal beings, etc). The RAW is clear IMO: it should make the illusion faint in mist of the cases, but I don't think that's RAI

[Man_Over_Game]

I'm mostly curious about the edge case of arrows and rocks, especially in cases where they could pass through (fake water fall, incorporeal beings, etc). The RAW is clear IMO: it should make the illusion faint in mist of the cases, but I don't think that's RAI

I think it has to work like this:

Creature sees physical interaction with illusion
\/
Creature discerns it is an illusion => Illusion becomes transparent to that creature.
/\
Creature uses an Investigation check for anything suspicious


So, from my understanding, the creature would need to see something in the rocks moving through the illusion that doesn't seem natural, which may not be something that they noticed right off the bat. You can use Passive Perception, Passive Investigation, advantage on their Investigation check, or even just consider that's how a creature uses the Investigation check in the first place. Use whatever seems to work, but that's how I'd play it.

[Mjolnirbear]

I'm not really sure what's vague or unclear about Minor Illusion and physical interactions:



Physical interaction = Investigation Check. They're just two different ways of reaching the same goal.

What's vague is what constitutes physical interaction. Do they mean the NPC touching the illusion or throwing matter at it? Because if it's the latter, which a strict reading would support, it means no illusion ever works, because everything is matter. The air is matter, dust is matter, it can't even seamlessly touch a wall to match up properly without fizzling.

I don't believe in using science in magic but matter is everywhere, except the astral plane maybe. There are viruses and dust specks and fungal spores floating in the air.

I can't believe they meant the spell to be useless. But they don't define physical interaction, so that little phrase means whatever your DM thinks it means. So like other such rules, it depends on what your DM says, because the RAW says physical interaction, but utterly fails to define that.

[Aimeryan]

What's vague is what constitutes physical interaction. Do they mean the NPC touching the illusion or throwing matter at it? Because if it's the latter, which a strict reading would support, it means no illusion ever works, because everything is matter. The air is matter, dust is matter, it can't even seamlessly touch a wall to match up properly without fizzling.

I don't believe in using science in magic but matter is everywhere, except the astral plane maybe. There are viruses and dust specks and fungal spores floating in the air.

I can't believe they meant the spell to be useless. But they don't define physical interaction, so that little phrase means whatever your DM thinks it means. So like other such rules, it depends on what your DM says, because the RAW says physical interaction, but utterly fails to define that.

Yup. I also question whether physical interaction is possible at all - its an illusion. If physical interaction is not possible then nothing physically interacts with it, so the clause becomes moot.


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.

[GloatingSwine]

What's vague is what constitutes physical interaction.

I think that's inevitable because what constitutes physical interaction is going to change depending on what the illusion is of.

For instance if you fire an arrow at an illusion of a wall and see it pass through, that's enough to reveal that the wall isn't really there.

If the illusion is of a wall of fire though, and you fire an arrow at it and it passes through, well that's what you expect and so it doesn't reveal the illusion.

[Tanarii]

I'm not really sure what's vague or unclear about Minor Illusion and physical interactions:



Physical interaction = Investigation Check. They're just two different ways of reaching the same goal.
Your bolding only emphasizes one of two possible interpretations of the RAW. If you have to selectively piecemeal bold it's a pretty good indication you're making a personal interpretation and their is something vague or unclear that a DM needs to rule on.

[Fat Rooster]

The thing that gets me is that impossible things happen all the time in D&D land, even without illusions. Your arrow goes through the jelly, and it doesn't seem to care? Could be an illusion, or could still be a jelly monster. The orc seems to walk through the wall? Could be an illusion, or the orc could actually be blending into the wall. Could be a magic wall that you can walk through, or could be a magic orc.

Personally I rule that even knowing something is an illusion is not enough to see through it, that always requires a save. Trying to look through an illusion is how you get a save, sort of like those 3D images that you have to squint to see. Even if you know it is there, and what it is, you can't see it until you can see it. Everyone gets one free save against an illusion, (though they need a reason to be trying to look through it) and can take actions for more. A save lets you see through it for a min (or combat), at the end of which you get your free save back. The caster gets advantage, but does not automatically see through their own illusions (more of a balance thing to prevent minor illusion 'invisibility' abuse than because it makes sense).

Imagine having those hidden 3D images on your wallpaper. People are unlikely to notice them, as they have no reason to. As soon as somebody points them out, some people will instantly be able to see them. Some people will have to take a few seconds of squinting. Some people will take ages.

The fog case is one where you are almost always automatically trying to look through the fog, so everybody gets a save except in rare cases. An action searching for enemies in the fog is basically an action trying to look through the fog, which gets you a save. If the fog is light and you can already see an enemy well, you would not get a save spending your action attacking them. If you look out over a landscape and see a fog clouds, one of which is an illusion, you don't automatically get a save. You have no reason to be looking for something in that fog cloud. A lookout in conditions that don't favour fog might try to squint to see if they could make out anything in the fog moving towards them, because even if it is not an illusion they still want to try to see if there is something in it. They would get saves.

Nothing like RAW, but I think fits RAI. Gives a simple question for dealing with corner cases, in "Are they trying to look through it?". A tiger swinging at an illusion would probably look at it's paw when it doesn't make contact as it should, so even though it has no concept of an illusion it would still get a save to realise that it isn't 'real'. Low level illusions might additionally shatter when interacted with physically, but that would be a separate effect.

[redwizard007]

I'm mostly curious about the edge case of arrows and rocks, especially in cases where they could pass through (fake water fall, incorporeal beings, etc). The RAW is clear IMO: it should make the illusion faint in mist of the cases, but I don't think that's RAI

I really think you, and several others, are reading too much into this. When something interacts with an illusion, IT has a chance to disbelieve the illusion. Random leaves, dust motes, and quantum particles can disbelieve all they want and it will have no effect on my game. If a CREATURE observes an interaction, then it could make an investigation check, leading to disbelief. The keys are that random matter has no effect because it isn't making an investigation check, the illusion isn't dimmed for everyone just because one creature succeed at an investigation check, just for the investigator, and that common sense has to be used when ruling on illusions.

If I cast an illusionary stone wall in the middle of combat it is perfectly reasonable for everyone to assume it is a real wall, a magical creation, but still solid. If someone stumbles into the wall, takes cover against the wall, has a poorly aimed arrow go through the wall, or otherwise has a realistic (relatively speaking) opportunity to observe that the wall is a figment, then they may make an investigation check to confirm their suspicion. One could even make the case that a mage known for illusions might promt disbelief anytime he casts a spell, but it would still need confirmed with an investigation check. If confirmed, THAT INDIVIDUAL knows it is an illusion and can see through it relatively clearly. They may also inform their friends, but those friends would still need to confirm the illusion for themselves with an investigation check, probably with advantage. Obviously, spells with specific rules could trump those assumptions, but that is how I would rule.

[RSP]

Isn’t the Investigation check needed regardless? Per the RAW:

“When you look around for clues and make deductions based on those clues, you make an Intelligence (Investigation) check.”

I read upthread the example of the flag not properly reacting to the wind. A Player may make the deduction from that clue the flag is an illusion, however, to find out if the character does? That’s an Intelligence (Investigation) check.

Similarly with any illusion. “That arrow went thru that wall” is a clue. That the wall is an illusion is a deduction. To see if the character arrives at the deduction based on the clue, that is an Intelligence (Investigation) check.

[Skylivedk]

I really think you, and several others, are reading too much into this. When something interacts with an illusion, IT has a chance to disbelieve the illusion. Random leaves, dust motes, and quantum particles can disbelieve all they want and it will have no effect on my game. If a CREATURE observes an interaction, then it could make an investigation check, leading to disbelief. The keys are that random matter has no effect because it isn't making an investigation check, the illusion isn't dimmed for everyone just because one creature succeed at an investigation check, just for the investigator, and that common sense has to be used when ruling on illusions.

If I cast an illusionary stone wall in the middle of combat it is perfectly reasonable for everyone to assume it is a real wall, a magical creation, but still solid. If someone stumbles into the wall, takes cover against the wall, has a poorly aimed arrow go through the wall, or otherwise has a realistic (relatively speaking) opportunity to observe that the wall is a figment, then they may make an investigation check to confirm their suspicion. One could even make the case that a mage known for illusions might promt disbelief anytime he casts a spell, but it would still need confirmed with an investigation check. If confirmed, THAT INDIVIDUAL knows it is an illusion and can see through it relatively clearly. They may also inform their friends, but those friends would still need to confirm the illusion for themselves with an investigation check, probably with advantage. Obviously, spells with specific rules could trump those assumptions, but that is how I would rule.

Where do you get it from that I read too much into it? I'm gathering different interpretations to arrive at a playable ruling. As a DM, I'd probably go close to yours. My own DM's first reaction was basically any kind of touch by a physical object would render the illusion faint to anyone who observed said object touch the illusion (even in the case of a rock being thrown at a ghost). Playing an illusionist that wasn't great news. I also knew, I'd come across as highly biased and that the DM usually reacts well to arguments, hence the thread :)

[Segev]

The trouble arises, as others have said, from the clauses that say physical interaction automatically reveals it. Stepping away from corner cases, the obvious intent is that seeing somebody walk through a "solid" wall, or punching an illusory orc in the face and having your hand pass right through, should make it so that you don't even need an Investigation check to say, "Ahah! This is not really here!"

On the other hand, the Investigation check is precisely what is supposed to model the effort to notice the flag not flapping in the breeze you're feeling, or the wall having grass poking out of its base and moving a bit with the trembling of the ground in ways that indicate the wall isn't actually holding it still, or that the orc's not leaving footprints as it charges you.

The edge cases are things that people do deliberately or semi-deliberately to "test" the illusion. Especially if, as some spells intimate is possible, the caster causes the illusion to react appropriately (the orc dodges the spear; the wall cracks apart in front of the charge of the horse running through it (maybe it was flimsy or the horse was magically powerful); the ghost fuzzes and flinches from the sword you just swung through it). If you shoot an arrow at a wall, can you TELL it went through, and didn't just bounce off and fall to the ground (especially if there's any ground cover at all)? Make an Investigation check to determine it. If you sweep your sword through a bit of mist, can you tell if the mist's swirling is "right" for how you swung it? Make an Investigation check to judge.

The major problem, for a player Illusionist, is that DMs know when it's an illusion and when it isn't, a priori. This thus makes them far more likely to "test" the illusion and have the monsters "discover" it, or have the monsters just...ignore it...because it's not actually a threat and thus "testing" it by ignoring it is obviously fair and totally not making it a wasted spell. The tendency for illusions to be game-changers in encounters if they're believed also influences this.

A DM's illusionist has the advantage that players' characters only know what the DM tells the players they see, hear, etc., and thus if the DM tells the players that they see a wall of fire spring up in the distance, the players have incentive to stay out of the fire's heat. If the player's illusionist creates a wall of fire illusion, the DM knows it's not real and has to actively ask himself whether his monsters would approach it bravely if the DM knew it was real.

It's hard for even the best DMs; not all DMs are even good enough to be willing to make that choice rather than play ruthlessly.

[mrairtraffic]

Hello Playground,

So I have a question. The RAW is quite clear in this text (example from Silent Image):

"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."

But, how would you rule the illusion of something immaterial: fog, poison cloud, misty vampire, ghosts, etc?

Certainly the power of illusions can be dramatically limited by both the players' imagination and the DM's acceptance. I personally don't see any need to alter RAW as either the DM or player. I think what is most important is to have an open dialog early on about illusions and establish an equality. It is a tool that both players and DM's can use so why not just stand with RAW and take the good with the bad on both sides of the screen?

For example: As DM i was blessed/cursed with an illusionist that loved to do battlefield control using illusions nearly every combat. Minor illusion was his answer to nearly everything. Initially it was irritating because a clever player can simulate a lot of answers with just one spell. He walled off enemies, created cover, simulated difficult terrain ect. That said, I adjusted as the DM and realized that every illusion created changed the environment of the battlefield for the players as well as the mobs. If he created the illusion of a curtain raising in front of archers to block their field of view, those mobs would split movement, fire away and jump back into full cover from the rest of the players. Its easy to rule that the illusionist can auto-succeed disbelieving the illusion and can see through the curtain but there is no rule that party members could. This can lead to it's own issues, if the illusion creator fires an arrow through the curtain because he still has full view of the target, I would say that counts as physical interaction for the creature influenced by the illusion, assuming they are smart enough. In this way, it's not necessary to home brew rules such as 'fainting' or degrading the illusion. I find it's pretty easy to settle any disagreement at the table when the answer effects the player and the DM evenly. Frankly, it is awesome for the story narrative to have a player trying to be as creative as possible.

Another example for fun's sake: I was a new DM when I had this illusionist in my party. I accepted RAW for several sessions but was irritated feeling like this guy was nerfing every encounter I had. So as any vindictive DM would, I gave them a taste of their own medicine. A small dungeon delve lead to a boss fight. The party stealth up a set of double doors. The doors open to a large evil church and the entrance opens into a funnel web. The players have about 10x10 area inside the door that they can get into without webbing but the webs extend above and beside them. They move their minis inside the room being careful not to interact with the webbing because they detect three large spiders inside the room and deduce that touching the webbing will alert them of their presence. The spiders open combat by slinging their web ability at them and they continue the encounter trying to determine how to deal with ranged attacks when they are 'pinned down' at the entrance and the spiders won't advance on them. In context it's probably obvious that the original funnel web was an illusion. An illusion that the players didn't want to have any physical interaction with. Any time they fired through the web, I rolled perception behind the screen to determine if they disbelieved. Failure made sense to them, it's not impossible to fire a crossbow through webbing. It took three rounds of combat before they perceived the reality of the situation ...which in 5e is a lot of action economy for the bad guys and they were in a dire straights by the end of the encounter. I wouldn't say this is a great example of RAW but it was in accordance to how the table all expected their own illusions to work. My players' really enjoyed the challenge and the cheeky 'pay back' for trying to cheese everything. This encounter is what lead us to agree to the 'ground rules' of illusions and stick with RAW as best as makes sense. If the player yells to his party that he is making the illusion, any intelligent creature gets the same chance to disbelieve as the party. Very dumb creatures may not understand what an illusion is or assume that something completely unrelated is an illusion. Either way, for a DM is allows great opportunities to make a routine encounter much more interesting and fun by feeding off the players narrative. It's almost like an instant chance for the DM to convey some information about his monsters' desires, fears, moral, intelligence, ect. It's up to the players to decide what those reactions mean.