In my world, there is a race of shapeshifting creatures, capable of convincingly looking like normal humans in order to lure them and prey upon them. These creatures, however, are non-sentient and non-conscious, a bit like the aliens in Peter Watts' Blindsight. So what do they use to trick their human preys? They use telepathy.

These creatures can read minds and can therefore act like their prey thinks they will act. If one of these shapeshifters takes the form of a person and you stumble upon them, they will act like you expect, hypothesize or hope for, all depending on what you are actively thinking in that moment. They can handle grammar and syntax just fine by talking like you form words and sentences in your mind, and they can process all the needed info just as fast as you think them.

Their telepathy also allows them to pick up on the preferred answer that their target hopes for, so if asked "are you a shapeshifter?" the answer will be invariably negative, unless you want them to be one (in which case you probably just want to commit suicide-by-shapeshifter, because no one hopes to meet one). Their answers will always be soothing, reassuring or enticing, simply due to them being "programmed" to go for the answer that makes you feel the least fear and the most curiosity or attraction. At the very least, their answer will be as non threatening as possible given the current thoughts of their prey. Basically their telepathy allows them to communicate a bit like a Chinese room, again, just like in Watts' novel.

They have a few limitations, chiefly that they can only use their telepathy on one mind at a time and that they can't read anything other than what the person is actively thinking. However keep in mind that it's borderline impossible to have a conversation with someone without forming expectations or thoughts on their answers in your mind, which the shapeshifter will immediately use to its advantage. They can only shapeshift into humans or human-like forms, both by imitating a person they have seen or preyed upon or by picking something out of the mind of their next victim if they are thinking about someone in that particular moment.

Other than this, assume that their telepathy can't be blocked or jammed in any way and that their shapeshifting abilities are perfect, so that no physical evidence can be used to unmask them. However, people do know that they exist, though these creatures are not common enough to be a constant worry. Enough to get suspicious if someone acts strangely or looks out of place but not enough to be completely paranoid.

In answering this question assume that no relevant technology is available and that no large scale countermeasures are possible, such as anti-shapeshifter laws and customs, humans have only their own individual wits on a case-by-case scenario to save them from becoming a shapeshifter's next meal. Also consider that shapeshifters only target humans that are alone or with just another person, so they never approach more than 2 people. If the shapeshifter manages to be approached at social distance for more than a few seconds, there's no escaping, the shapeshifter will easily overpower up to 2 humans.

With all of this said, what kind of tactics and tricks might a human pick up to spot a prowling shapeshifter effectively?

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    $\begingroup$ How can a telepathic being being non conscious and non sentient? $\endgroup$
    – L.Dutch
    Commented May 19, 2019 at 17:06
  • $\begingroup$ In the same way your laptop doesn't need to be conscious or sentient to work, just imagine that to these shapeshifters your thoughts are like an input they are programmed to react to, just like your typing on your keyboard makes your computer do different things according to its software. $\endgroup$ Commented May 19, 2019 at 17:08
  • $\begingroup$ Just tried typing into my laptop "be a nymphet", it's not transformed into one. Nor did it do so when I decided I wanted a Pizza. Since the brain (particularly the male-brain) thinks about various erotic images/scenarios when, that is, not distracted by thought about various erotic images/scenarios - how could this work? $\endgroup$ Commented May 19, 2019 at 17:15
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    $\begingroup$ @InquisitiveGeek Links can be broken, please take the time to edit your question to include the salient information. as per forum advice in the help center. $\endgroup$ Commented May 19, 2019 at 17:23
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    $\begingroup$ Disappointing that this was closed as "unclear what you're asking" when the question seems clear enough to get a bunch of upvotes and a handful of relevant answers. $\endgroup$ Commented May 20, 2019 at 13:07

5 Answers 5


One way to spot such shapeshifters is a variant of the shibboleth, in which you ask a question whose answer you ignore, but are able to easily verify once it's given.

For example you can ask the stranger to tell you two numbers and their sum, provided you aren't able to make the sum in your head yourself. A shapeshifter will not be able to come up with these numbers (much less the sum), unless it reads them off your active thoughts, which means that you know the numbers, so they're not valid:

"Tell me two numbers and their sum" (thinks "Sixtyfive and ninetythree is... er... one hundred sixty-eight? No, wait, fifty-eight") Hears "Thirty-four and seventy-seven is one hundred eleven": okay. Hears "Thirty-four and sixty-one is nineteen hundred": Haha, funny. Try again. Hears "Sixty-five and ninety-three is (one hundred)fifty-eight": STOP THERE!

Once you hear the two numbers - meanwhile, you concentrate hard on adding some other number - you can verify the sum.

Again, most people will quickly pick a favorite pair, which means that they'll have a competing thought in their minds and think of the numbers they would choose if asked - but the chances of two people choosing the same pair are very slight. If someone utters the same numbers you were thinking, ask them to choose different numbers (and think of the same numbers). If they choose the same pair again, that's something only a nonsentient shapeshifter would do.

Whoever fails three sums in a row has to be a shapeshifter, or really really bad at math.

If you hear someone nearby, asked by someone else, uttering the same numbers you were thinking of, just say that aloud. In case, only ask someone after everyone else has moved out of their range.

Another better possibility would be to carry two large wooden dices, and throw them on the ground without looking at them, then asking the stranger to read the result, and finally check.

Or a box with a deck of large cards. Throw the box to the stranger, and tell them to shuffle the deck, then look at each card, announce it, and then show it to you. The shapeshifter won't be able to recognize the cards, nor to read them in your mind, and it cannot guess without the virtual certainty of being immediately caught. If it says "Queen of Hearts" and shows you a jack of spades, well.

In all these cases the one being tested must be the only one to see the test until it's complete - for this reason, the card deck is better. But again, a protocol would quickly be established so that everyone would know what to do if asked to pick up a card.

  • $\begingroup$ You could just request an open ended question: suspecting him a shapeshifter, he said "I say sir, can you name a prime, perhaps three or four?" This is kind of like first gen cryptography. $\endgroup$
    – user458
    Commented May 19, 2019 at 19:23
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    $\begingroup$ @fredsbend the problem there is that the answers you want are going to be right there to be read back at you. You need to be able to ask a question to which you don't know the answer, hence why dice or cards are useful. $\endgroup$ Commented May 19, 2019 at 20:08
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    $\begingroup$ That's it, perfect answer. I apologize for the delay in accepting it, I've been busy elsewhere, plus I was slightly turned off by my question being closed as unclear despite the fact that quite a few people understood perfectly what I was asking and provided eloquent answers. $\endgroup$ Commented Jul 7, 2019 at 14:37

A deck of ordinary playing cards seems to be a pretty good solution. Give the deck a few good shuffles every now and then so the order is completely randomized and you have no way of knowing the order. Then, if you're having a conversation with someone that you aren't sure of, deal 'em a poker hand and ask them to name the cards they have. (You may want to get a safe distance away after giving them the poker hand, and possibly ready your non-anachronistic lethal weapon of choice.) Imagine a royal flush while doing so, or really any given poker hand of five cards. When they name their hand, just riffle through the deck. If they're human, they'll name the five cards that aren't there. If they aren't - well, then they've got a 1 in ~ 2.5 million chance of naming all five cards correctly, because those are your odds of correctly guessing those five cards. (My math might be a bit off - I just did a back of the envelope from poker odds.)

Now, if you've got tech or magic, a talking 8-ball is even better. Rig up one that randomly generates a color out of eight given (primaries and secondaries, plus black and white) and then will loudly announce the color after a five second pause. Toss the ball to the suspected NSNCTS and run about six rounds of it. Should only take half a minute, and the odds are 1/ 262144. The idea here is to give the target easily confirmed information that only the target knows at the time.

  • $\begingroup$ To make things faster: just use Hearts so you have fewer cards to run through. Bonus points if you imagine a straight flush of spades and they fall for it. $\endgroup$
    – Joe Bloggs
    Commented May 19, 2019 at 19:08
  • $\begingroup$ I'm assuming the worse case scenario here, in which case you can't throw them off by merely imagining what you want them to say if it's clearly wrong, i.e. asking them the color of the sky, and then imagining the sky is red. Same thing with using an all-hearts deck and imagining a spades hand. But I suppose if it works, it'd be much easier. $\endgroup$
    – Halfthawed
    Commented May 19, 2019 at 19:10
  • $\begingroup$ That’s why it was the bonus point scenario. Mostly I meant that checking through 8 cards for the stated 5 is faster than checking through 47. :-) $\endgroup$
    – Joe Bloggs
    Commented May 19, 2019 at 19:14
  • $\begingroup$ At that point, it's really just a matter of preference. Odds of correctly labeling 5 picked cards out of 13 is 1 / 1287. If you feel comfortable betting your life on those odds, go ahead. $\endgroup$
    – Halfthawed
    Commented May 19, 2019 at 19:20
  • $\begingroup$ Seems easier just to use number cards, deal a couple without looking at them and ask the to tell you the sum. Once they've done that, they can turn the cards over and you can verify. No need for the hassle of shuffling through an entire deck of cards in close proximity to a possible killer monster, and no need for probabilistic shortcuts. $\endgroup$ Commented May 19, 2019 at 20:11

Assuming they can't do higher level reasoning by themselves:

  1. Ask someone to curate a list of questions that you have never seen before. These could be combination of math questions like 10x43=? and open ended questions like "how can we solve world hunger?", captcha style questions like "how many oranges are in this photo?" Always keep this list with you, but never look at it.
  2. When you think someone may be a shapeshifter, give them the list. Ask them to choose any 3 questions without telling you and write down the thought process/ answer somewhere. When you review the answers it should be clear if they are capable of independent thought or not (correctness does not matter, just the thought process)

Not sure if phones/computers are acceptable technology in your world, but it is simple to make an app specifically for this that generates random questions/captchas and can check the answers (no human involved = no chance of reading thoughts).

It is important that you don't know the problem already, since you might imagine the method to solve it, which they might read and follow. Imagining "the wrong answer" may not work either since your thoughts contain the information that the answer is wrong so they won't choose it (it is impossible to block your own thoughts about it). The only solution is to truly be unaware of the problem itself.


people do know that they exist

I cannot imagine how the existence could be discovered of a predator whose total design is so conveniently ironclad.

I imagine a person that suspects they're under threat would deliberately fabricate a fake person and then concentrate on that fake person while traveling through insecure areas. A community would immediately retreat into secure, isolated areas.

So, forgetting the absolute implausibility of how this could even be true: you are suddenly put on notice that one of these killing machines is in your area. You know, like an Amber Alert. What do you do? Well, your house is safe, so you can just keep the door locked and stay indoors. But you eventually have to get groceries. Before leaving, you pull out your book of Leeroy Jenkins fan-art and study it carefully until you have a really solid picture of him in your head. Then you go outside and hurry to the store, concentrating on Leeroy and how badly you hope to meet him, and pushing all other thoughts out of your head.

If a shapeshifter becomes aware of you before you notice it, it will shift into your picture of Leeroy Jenkins. If you meet Leeroy Jenkins, you will kill him.

If you encounter any other non-humans on your route, all of you will stay away from each other out of mutual distrust. If anyone approaches you, you will kill them.

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    $\begingroup$ Wouldn't the shapeshifter just make sure to not appear to be Leeroy, then? Seems the shapeshifter would pick up on the fact that the reason that you're thinking about Leeroy is to kill him, hence they'd not look like him. $\endgroup$
    – user45266
    Commented May 19, 2019 at 22:57

How smart are they?

If you ask them a question that you don't know the answer to yet, but can figure out if you choose to, you're golden. Examples:

"Hey, what's behind me right now?" If they're a dumb shapeshifter (and they're not sentient, so they probably are dumb), they won't really know the answer. Then you turn around and verify their response.

"Pick a tree branch, and tell me how many leaves are on it"

"Go over there, and name as many objects as you can in that room"

Or the classic "How many jellybeans are in the jar"? (tell them to open the jar and count)


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