Undead Don't Have Special Eyesight

From time to time the argument crops up that undead can see through all illusions. There are two main ways that people come to this erroneous conclusion.

    Premise #1: Undead are immune to mind-affecting effects.
    Conclusion #1: Since all illusions are mind-affecting, including invisibility, undead are immune to all illusions.

    Premise #2: Many undead don't have eyes (skeletons have empty sockets, zombie eyes are rotten, incorporeals are just ghostly, etc.).
    Conclusion #2: Since they don't have eyes to receive light, they must be sensing things in other ways, probably with some kind of life-force sense, and since illusions don't have life-force, undead see right through them.

Oddly enough, both premises are true, but both conclusions are false. Let's tackle these one at a time.

    Premise #1: Undead are immune to mind-affecting effects.

True. It's right there in the description of the standard undead abilities: "Immunity to all mind-affecting effects (charms, compulsions, phantasms, patterns, and morale effects)."

    Conclusion #1: Since all illusions are mind-affecting, undead are immune to all illusions.

Whoah! Hold it right there. Not all illusions are mind-affecting! Figments and glamers produce unreal but certainly not mind-affecting effects. If you use silent image to make an illusion of a cottage, the cottage may not be physical, in that it can't shelter you from rain, but it's also not simply a magical image in your mind. You can disbelieve it, which mean you recognize it is false image, but it doesn't disappear (notice the spell says "Will disbelief" not "Will negates."). A lot of people use a hologram as an analogy for figments (and that's a pretty accurate comparison if you don't nitpick it too much) ... and clearly holograms aren't in your mind. Shadow illusions draw upon extradimensional energy to create things that are semi-real (more than just images or sounds), so they're not just mental effects either. So the first part of the conclusion here is false. Since the first part of the conclusion is false, it can't be used to prove the second part of the conclusion.

So far: One "proof" that undead can see through illusions is shot down.

    Premise #2: Many undead don't have eyes.

True, and it's obvious by observation, at least as much as we can't in the real world: a skeleton doesn't have eyes, and a rotting corpse's eyes are usually in no shape to be used for vision. While we have no incorporeal creatures in the real world, it's not much of a leap to say that creatures that don't have physical bodies don't have physical eyes.

    Conclusion #2: Since they don't have eyes to receive light, they must be sensing things in other ways,

This conclusion is somewhat shaky. Nothing in the descriptions of the undead presented in the Monster Manual say anything about them sensing things any differently than living creatures.* In fact, Sage Advice in Dragon Magazine clarified** that unless otherwise stated, all creatures have the normal five senses of humans (sight, hearing, smelling, touch, and taste). None of their descriptions say they have any unusual sense (unless they have something like blindsense or blindsight listed). In fact, no creature in the MM that lacks eyes (like elementals) is listed as having any special form of sight ... why is it that only undead get this "other ways" sense?

D&D is written for humans. It's written by humans from the perspective of humans, and when comparisons are made, they're made to a baseline human. Things that aren't outright stated in the D&D should be assumed to be human-normal. Huge parts of the game are built around the human as the standard, from armor class (the default AC of 10 is the AC of your average unarmored human) to attack rolls (your average unarmed human with no special training has about a 50% chance -- 10+ on a d20 -- of hitting another average unarmored human with a punch) to saving throws (default DCs are set according to what your average human could resist, dodge, or survive) to skill checks (DC 10 is something your average unskilled guy could succeed at about 50% of the time). With this humanocentric view, it should be clear that if there is no listed answer to a question, the answer almost certainly is the same as asking the question about a human.
    How do bugbears poop? Just like a human.
    Where do gnomes have body hair? In the same places humans do.
    How good is an aboleth's sense of smell? About as good as a human.
    How spicy is too spicy to an aasimar? About as much as a human would consider too spicy.

Of course, this comparison doesn't hold up to creatures that obviously resemble nonhuman real-world creatures. If asked about the sense of smell or taste preferences of a pegasus, I'd compare it to a horse. If asked what sort of meat owlbears prefer, fish or chicken, I'd find out what real bears like. But for undead, the closest comparison is to humans, since most undead are made from humans (or other humanoids, which bring the comparison back to humans again).

This is really an aspect of Occam's Razor:
    "The simplest of two or more competing theories is preferable and that an explanation for unknown phenomena should first be attempted in terms of what is already known." - Dictionary.com

So, in this case, if one explanation is "undead senses default to the human norm through some process that mimics human senses" and the other is "undead have some strange method of sensing their environment, even though no part of the rulebooks says that undead have this ability", clearly the first explanation is the simpler one and is probably correct. When backed up by Sage Advice's statement that creatures have normal human senses unless otherwise stated, the evidence is strongly in favor of the familiar human senses rather than the kooky they-have-it-but-it's-not-mentioned-and-only-described-under-one-undead-creature sense.

One more comment on this point: Of course, all undead do have a special sense that humans don't have--darkvision. You know this not only because the undead type entry says they have it, but all undead in the MM do (or at least should) have darkvision listed. But the rules also say that darkvision is "black and white only, but is otherwise like normal sight." "Normal," of course, means "like a human's" in D&D. And if undead have a special sense, why would they need darkvision which works like human sight? If they have senses that aren't like human senses and yet have another sense that is like human sight, isn't that strange? Again, the simplest answer is that their senses work like human senses and darkvision is a supplement to that, just like dwarves have humanlike senses with darkvision as a supplement.

     probably with some kind of life-force sense,

Here's where things get totally crazy.

First, if undead could sense life force, that would be a HUGE advantage. They could ignore invisibility, mirror image (which, mind you, is a figment, and as explained above produces a real image that creatures can actually see, not a mental trick), blindness effects***, blur, displacement, magical darkness, most concealment effects, know when a creature is standing around a corner (unless this "lifesense" requires a line of effect to the living target), spot hiding creatures automatically ... and on and on. Clearly the lowly skeleton is a much greater threat because of this! What evil necromancer or cleric in his right mind wouldn't keep some skeletons on hand to alert his living minions of invisibles and such in the area! We must increase the CRs of all undead immediately! After all, this lifesense operates like a combination of deathwatch, see invisibility, and true seeing!

But wait ... again we run into the problem that none of the D&D books say that undead ignore invisibility and so on. And this lifesense brings up a lot of questions. Can they sense nonliving things that aren't creatures, like walls? Deathwatch says they can recognize nonliving creatures such as constructs but doesn't say anything about recognizing nonliving objects, presumably because the default assumption of a human caster can already sense that a floor exists, but the lifesense argument cannot use "undead have human senses" as an assumption because the lifesense conclusion is trying to explain why undead do not have human senses.**** It would make it a lot harder for undead to get around if their only perception of their environment was through living or undead landmarks. Heck, if they can't sense objects, undead monks wouldn't be able to use Deflect Arrows since they couldn't perceive the arrow in flight or even as it was leaving the enemy's bow. Same goes for Improved Disarm and Sunder ... how can you knock a weapon out of someone's hand or attack the weapon if you can't actually sense the weapon in the first place? Heck, how can undead pick up weapons and use them in combat? They can't see a scabbard to attach to a belt (which they also can't see), and they can't see the sword in the scabbard. Undead must walk around with their weapons out all the time because if they ever put down their weapon they'll never be able to find it again! And good luck using magic items ... a wounded undead trying to dig a potion of inflict critical wounds (which it can't see) out of its pouch (which it can't see) so it can heal itself won't be able to read the label on the potion (since the label and paper are not living)!

(As an aside, notice the sample vampire in the MM has the Blind-Fight feat. Why would it ever need this feat if it could sense invisible creatures and operate in total darkness? You could argue that it had that feat while it was still alive and just has to "eat" the choice of a bad feat that does nothing for its undead self (just like the Diehard feat would do it no good, since as an undead it can never go to negative hit points). But why would WotC deliberately give a bad feat selection into a creature in the MM? Wouldn't that make the creature weaker than its CR would indicate? You don't see them giving Improved Unarmed Strike feat to longsword-wielding humanoids--which is just as innefectual (and dumb) a choice as the lifesensing vampire with the useless (to it) Blind-Fight feat. Similarly, the sample elite vampire has Blind-Fight and Improved Disarm, which as I point out about is problematic for the undead that can't sense nonliving things.

So maybe they can sense objects walls and floors and such just as well as a human can ... but if you're giving them that, why add in on top of that a special and superior sense that isn't mentioned in the books? If the whole point of lifesense is that undead don't see things like humans do, why turn around and say that they do see things like humans do and then slap some goodies on top of that--goodies that aren't mentioned in the core books?

Can you hear with lifesense? Taste? Touch? Smell? No answers. Wait, undead have to be able to hear, since their creator can give them verbal commands. So is that a quirk that they can hear only because it comes from a living creature? Why would this sense differentiate between sound waves made by a living creature and those that aren't. I guess lifesense lets you hear. Probably like a human. Which again makes me wonder why they need a special sense if they're already using human senses.

What about energy? Acid, cold, electricity, fire, and sonic aren't objects. Can lifesense detect a wall of fire? Can it detect heat at all? Spells?

And what range is this ability? 60 feet? 120 feet? As far as a human can see? This ability isn't defined anywhere. If it has a limited range, then the living should have long ago figured out what that range and exploited it. "An army of undead is on the way? Fear not, just stay 200 feet back and pepper them with arrows, they won't be able to sense where we are."

And let's talk a bit about animate dead. This is a 3rd-level cleric spell that not only crams negative energy into a body, but it gives that undead body this super-powerful lifesense ability! And the duration is instantaneous, which means it can't be dispelled. How come some smart spellcaster hasn't taken that spell apart to find the "give target lifesense" code, strip out the "cram in negative energy" part, and end up with a lower-level spell (since it does less than the original) that gives the target creature instantaneous-duration lifesense? My answer is that there is no lifesense component, that the default senses of any creature are human-normal (in fact, I'd let you create a human senses spell that gave the target human-level senses instead of its own normal senses, but for most creatures that would be pointless). Some people might argue, "no, see, the lifesense comes as part of the natural abilities of the undead life force you're drawing from the Negative Energy Plane." Ok, I can see that, but that still doesn't mean that someone couldn't make a spell called summon a little undead spirit to wrap around my head and relay to me what it senses, which would have the same effect as the give me lifesense spell. Yet no such spell exists.

It's easier and simpler to let undead have normal human senses rather than make assumptions about an undefined yet omnipresent-in-every-undead lifesense ability that you have to keep redifining and clarifying when it's never mentioned in the books in the first place.

    and since illusions don't have life-force, undead see right through them.

If undead have lifesense, and they don't sense things like humans do, then undead should see right through illusions. But hopefully I've shot the whole lifesense idea in the head, which means that the first part of the conclusion is false, and therefore it cannot prove that the second part is true. (It doesn't mean the second part is false, it just means that you have no evidence proving it true, and since the burden of proof is on the one insisting on the truth conclusion, it's up to you to prove me wrong. Any takers?)

Thus, barring any proof that they do, undead have no inherent ability to see through all illusions (though they are unaffected by mind-affecting illusions, which is most of them).

And they don't have lifesense, dagnabbit.

Footnotes:
* It has been pointed out to me (thanks, Charles G!) that there is one undead creature with a lifesense ability: the dread wraith. It tells you exactly what this power does. No other undead is listed with this ability, not even the regular wraith. So if this is the only undead with this ability mentioned and described, why assume that all undead have it?

** I would have thought they would have put this clarification into the MM 3.5 but it isn't there ... oh, well.

*** Isn't it funny that in 3.5 flare is evocation, blindness/deafness is a necromancy spell, but power word blind is an enchantment? It's like a Monty Python sketch. "No, really, you're blind, I insist." "Very well, then." (falls off cartoon cliff)

**** Note that the dread wraith's lifesense ability only mentions living creatures and the strength of their lifeforce. If you use that as the basis for your "generic" lifesense ability that all undead (in theory) have, it still doesn't address whether or not they can see nonliving things. It's particularly relevant to this rant that the one creature in the MM that does have a lifesense ability doesn't have any explanation of how it perceives the rest of the world (answer: the default answer of "just like humans").