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Assuming an illusion spell can create a working mirror:

If I use an illusionary mirror to look around a corner, a corner that neither I nor the caster of the illusion can see, what do I see in the mirror?

OR to phrase it another way, how do illusions deal with incomplete caster knowledge?

Inspired by the comments on the question Can you use Minor Illusion to create an illusion of a working mirror, i.e. with reflection?.

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    \$\begingroup\$ I'm still failing to see how this isn't a duplicate. In what way does the answer to that question not answer this question? \$\endgroup\$ – SevenSidedDie Mar 31 '16 at 0:06
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    \$\begingroup\$ Come to think of it, I should probably vote to close that other question as primarily opinion-based, as it seems clear that there's no definitive answer in the rules, and people are just putting whatever makes the most sense to them. \$\endgroup\$ – GMJoe Mar 31 '16 at 0:20
  • \$\begingroup\$ Or in other words, the answers to that question are the same as the answers to this question, because it's a dupe. \$\endgroup\$ – Miniman Mar 31 '16 at 1:10
  • \$\begingroup\$ Granted, this one is more simply worded, which is a virtue. \$\endgroup\$ – SevenSidedDie Mar 31 '16 at 1:25
  • \$\begingroup\$ @SevenSidedDie, you are the one who told me to ask a new question when I asked this query in a comment to the other question. Why are you now marking this as a duplicate? \$\endgroup\$ – Greenstone Walker Mar 31 '16 at 2:46
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The general rule for illusions is on p.203 of the PHB:

Illusion spells deceive the senses or minds of others. They cause people to see things that are not there, to miss things that are there, to hear phantom noises, or to remember things that never happened. Some illusions create phantom images that any creature can see, but the most insidious illusions plant an image directly in the mind of a creature.

So, what you see in the illusionary mirror is whatever the creator of the illusion wants you to see subject to the capability of the particular spell or other effect that creates the illusionary mirror. Some spells like Minor Illusion are static and limited in size so those limitations will impact on what can be shown, others like Phantasmal Image actually have the target "filling in the blanks" so that they autocorrect any errors in the illusion.

If the caster knows what a real mirror in that location and orientation would show then they can chose to show that. Irrespective of their knowledge they can show what they think is around the corner, or what they make up, or an image of the 347th layer of the Abyss, or a close up of a dental filling, or anything else you can imagine that you want to put in a mirror.

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    \$\begingroup\$ I don't think the text you've quoted necessarily supports your argument. Your interpretation is valid, it's just not the only interpretation possible - and illusions that don't react realistically to ambient light in the absence of the caster would be trivially easy to spot, and therefore not nearly as hard to detect as the rules seem to suggest. \$\endgroup\$ – GMJoe Mar 31 '16 at 0:23
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    \$\begingroup\$ @GMJoe I don't see that. Minor Illusion for example would still require "an action to examine" because that's what the rules say; you can now describe why your action revealed the illusion. What you couldn't do is make a Minor Illusion that showed a 100 foot corridor because "the object" which includes the reflection is limited to 5 feet. \$\endgroup\$ – Dale M Mar 31 '16 at 0:30
  • \$\begingroup\$ That's exactly my point: For an illusion to only be obvious upon deliberate investigation, it has to be reasonably convincing while not being deliberately investigated. When I'm walking down a dungeon corridor with a torch in hand, the light levels are changing visibly everywhere within my field of view, and illusions still require a deliberate investigation to discover - which means they must be reacting to the light emitted by my torch. (I'm not sure why you brought up the illusion of a corridor thing.) \$\endgroup\$ – GMJoe Mar 31 '16 at 4:51
  • \$\begingroup\$ GMJoe. Yet if the illusion is perceived as such, then the image becomes translucent, which proves that the illusion does not reflect light according to our expected laws of physics. Whatever "reflection" occurs to make the image seem realistic must have some other explanation. It may be analogous to Phantasmal Force, where the victim's mind rationalizes what he sees. Or it may be some other magical principle which is left to the DM to explain (or leave a mystery). \$\endgroup\$ – pokep Mar 31 '16 at 17:39
  • \$\begingroup\$ Dale M offers further evidence with his example of the 100' corridor. If an illusion worked according to the laws of physics, then an illusion could be a two-dimensional representation of a larger 3-d space, using the same principles as a trompe-l'œil painting. But the rules explicitly forbid this. There is no suggestion that such an illusion is simply more difficult to create or easier to perceive, it explicitly cannot be done. This strongly suggests that illusions work in some manner which interferes with the "natural" illusion created by the trompe-l'œil image. \$\endgroup\$ – pokep Mar 31 '16 at 17:46

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