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As part of the description of the Detect Thoughts spell, it states:

As an action, you can either shift your attention to another creature’s thoughts or attempt to probe deeper into the same creature’s mind. If you probe deeper, the target must make a Wisdom saving throw. If it fails, you gain insight into its reasoning (if any), its emotional state, and something that looms large in its mind (such as something it worries over, loves, or hates). If it succeeds, the spell ends. Either way, the target knows that you are probing into its mind, and unless you shift your attention to another creature’s thoughts, the creature can use its action on its turn to make an Intelligence check contested by your Intelligence check; if it succeeds, the spell ends.

My question is, how exactly does the target know it was you? I always assumed it was a sort of "psychic ping" in the target's mind that they can sense originates from you. But what if you were invisible? How would the target know you were the one probing into their thoughts? Would they just get a mental image of the person casting the spell, even if they couldn't see them?

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    \$\begingroup\$ @Eddymage there is nothing unclear. It asks for how two features interact. You're searching for something that's not relevant. Here is a possibility: somebody else cast Greater Invisibility and maintains concentration. Then the target casts Detect Thoughts. Does that make the combination work? Yes. Does it change anything with the question? No. \$\endgroup\$
    – VLAZ
    Feb 29 at 8:13
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    \$\begingroup\$ @Eddymage Neither 1. nor 2. depend on the exact mechanism how one ended up invisible and using Detect Thoughts. Only the interplay between the two. \$\endgroup\$
    – VLAZ
    Feb 29 at 8:52
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    \$\begingroup\$ @Eddymage I see your point. I think you're saying the question is actually "how exactly does the target know it was you?", and the invisible part is subsidiary. \$\endgroup\$
    – Jack
    Feb 29 at 10:31
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    \$\begingroup\$ @Jack Yep: indeed, the OP asks "My question is, how exactly does the target know it was you? I always assumed it was a sort of "psychic ping" in the target's mind that they can sense originates from you" \$\endgroup\$
    – Eddymage
    Feb 29 at 10:35
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    \$\begingroup\$ @Jack: Ring of Invisibility can work: 1. cast Detect Thoughts (1 min concentration). 2. use an action to become invisible (via the ring). 3. Walk out from behind cover or whatever and use your action to shift Detect Thoughts to a creature. 4. Use your action to probe deeper. There are of course tons of other ways to make this happen, including the broader scope of simply not being visible to the target, like perhaps mundane hiding, a peephole, them being blindfolded, blindsight, or using Detect Thought's ability to pick a nearby mind not based on sight of the creature. \$\endgroup\$ Feb 29 at 22:29

2 Answers 2

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You means you

As comments to the original post point out, maintaining invisibility while casting detect thoughts is difficult to do, but let's accept the premise that the target of the spell cannot see (or presumably hear) the caster, even though the spell requires the caster to both see the target and be within 30 feet upon spell casting and to maintain that proximity, if not visual contact, for the duration.

In this situation, what does 'the target knows that you are probing into its mind' mean?

You are more than just someone

Spell descriptions in 5e are written from the perspective of the caster, and one would be hard-pressed to find a description without the word 'you' in it somewhere. So it could be that 'knows that you are probing' just means 'knows that someone is probing'.

However, consider the description of sending:

The creature hears the message in its mind, recognizes you as the sender if it knows you, and can answer in a like manner immediately.

Here, rather than simply saying that the target recognizes you as the sender, the clause is added 'if it knows you'. This directly implies that if you target a creature that does not know you, it will know it is being contacted by someone, but will not know it is being contacted by you. It's a bit of a stretch, but this could further imply that by leaving out the 'if it knows you' clause, detect thoughts is not, in fact, implying the target knows only that 'someone' is probing their thoughts. Instead, in detect thoughts the target really does know that you are doing so.

Tangential support for this interpretation also comes in the description of the dream spell. Here, the caster chooses a messenger - be it themselves or another willing creature. The messenger then contacts the target of the spell. Nowhere in the spell description does it say that the target of the spell recognizes the identity of the messenger, whether known or not.

If intentional, what does this mean?

5e does not necessarily have a level of editing and rules consistency that allows us to trust that the differences between these descriptions are intentional. We can agree that 'spells do what they say they do' - but did the writers really tell us exactly what they do? If we suppose they did - or if you, as a DM, want to play that they did - then in the case of a target contacted by dream, they have no idea who is contacting them. With sending, they know that someone is contacting them, but only recognize their identity if it was already known. And for detect thoughts, they know the caster, regardless of whether they are familiar with them or not.

So what does it mean to know someone you do not recognize? For me as a DM, I imagine it is a bit like receiving a phone call from someone you do not recognize. Their number is saved on your phone, so you 'know' who it was, and were they to call again, you would recognize them as the same individual, even if you know nothing else about them. Similarly, the person subject to detect thoughts who cannot see or hear the caster, can still recognize them as an individual, even if they know nothing about them. If they later see or hear them outside the spell, they will instantly recognize them - "So you're the voice that was in my head!" Recognizing in real life the magical character that was heretofore only in your dreams is a common enough trope, and as a DM I would lean into that sort of experience.

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    \$\begingroup\$ So you're saying detect thoughts gives an image of the caster to the target? Or that the target hears their voice? But the spell doesn't say that. The DM can certainly decide that, but the spell doesn't say. There's no caller ID described. \$\endgroup\$
    – Jack
    Feb 29 at 10:28
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    \$\begingroup\$ @Jack The spell says the target knows the caster. It is left to the DM to interpret what form this knowledge takes. I imagine it like caller ID - here is a particular entity that is contacting me, and I would recognize it in the future. YMMV. \$\endgroup\$
    – Kirt
    Feb 29 at 17:45
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    \$\begingroup\$ You've touched minds with the target and the target has gained identity insight of your mind. Whether or not this means the target will recognize you if it doesn't already know what you look like is an open question. \$\endgroup\$
    – Joshua
    Feb 29 at 18:31
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    \$\begingroup\$ @Joshua Yes, for me that kind of depends on the nature of the dramatic arc and where I want the 'reveal' to happen. \$\endgroup\$
    – Kirt
    Feb 29 at 18:42
  • \$\begingroup\$ @Jack: I would rephrase "image" to some further unexplained manner of identification. I would interpret this that the creature would definitely be able to confirm that the same person detected their thoughts twice (instead of two different people), but that doesn't necessarily mean they know your name or what you look like. \$\endgroup\$
    – Flater
    Mar 1 at 4:23
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The DM will need to adjudicate exactly what "you" means

The spell does not specify any information about you that is given to the target

It is easy to read the sentence as meaning, "hey, that dragonborn over the corner is trying to read my mind!" But the spell does not specify what information the target learns about the caster, so the DM will have to decide.

The spell does actually specify one additional piece of information. It knows "you are probing into its mind", and that's all. The spell description specifies nothing else; not your appearance, not your voice, nothing.

It would make more sense if it said "someone", instead of "you"

The spell would make more sense if it said "someone", instead of "you". As an analogy, if you throw a rock at someone's window, they know someone did it (or make that reasonable assumption), but they don't know who unless they have additional information. For instance, if they look outside and see you, they can make the assumption that it was you, even if "you" just means "that dragonborn standing in my front yard."

But, since the spell does not specify any additional information, it does not supply any, and so the target does not know who "you" is.

The DM could decide to provide information, but they would have to decide what

The DM is free to decide differently, that the spell provides some sort of identifying information as to who "you" is, but since the spell doesn't say what sort of information, the DM would be adjudicating that, and would need to decide what additional information to provide. And that's completely okay, but it is based on the DM's interpretation, and not the spell description.

The target may be able to make some guesses

It is reasonable for the DM to consider additional information that the target may have, depending on circumstances, and on the knowledge and experience of the target.

In the case you presented, they may reasonably think that perhaps there is someone nearby who is probing their thoughts. But in the end, the DM will need to decide.

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