How Does Detect Even Work?

How Does Detect Even Work?

So, I got into another rules argument (first with a friend, then with myself) that left me buttblasted and made me once again realize one of the flaws of GURPS - it's been made by multiple authors, like Freddy Krueger, the bastard son of 100 maniacs. My friend pointed to one of my Detect-based spell builds and said "What the hell is going on here? First it says that it detects every appropriate object nearby, but then  it says that a successful roll detects the nearest one." I already started typing up a response, saying "Eat sh-" but then I thought "Wait a second, he might be right. I better research the topic before telling him to kick rocks." Oh boy, I was expecting a mess, and my expectations were exceeded.

The description of the Detect advantage clearly says: "On a success, the GM tells you the direction to the nearest significant source of the substance, and give you a clue as to the quantity present. On a failure, you sense nothing."

So, what's so difficult to understand here? Nothing, actually. It works well as is. Then GURPS Powers throws a wrench in the works. While it is one of the most coherent and airtight written GURPS books (that people don't read aside from the advantage chapter), it has some minor flaws - premade abilities have very short descriptions that often skip over important details of the underlying advantages. For example, the Frog ability doesn't elaborate on whether the transformation time is instantaneous or takes 10 seconds as per Alternate Form, and this is very important. In our case, let's take a look at Telescan on page 150.

"Detect (Sapient Minds; Analyzing, +100%; Long-Range 1, +50%; Precise, +100%) [70]. Notes: Detects sapient minds at great distances. A successful Sense roll gives the location and race of everyone nearby – handy..."

Or how about Electronic Support Measures on the same page?

"Detect (Radar and Radio; Precise, +100%; Signal Detection, +0%) [20]. Notes: On a successful Sense roll, this ability detects radar and radio signals, and reveals the distance to each source..."

Wait, where did the nearest subject go? This is not "skip over important details", this is outright "rewrite important rules, and in a significant way." Is there another book with premade abilities that are described in more detail? Of course, there is GURPS Psionic Powers (that, admittedly, is not without flaws). The book has several Detect-based abilities, but they all detect the nearest subject, just like how GURPS Basic Set instructs.

Anything else? Let's check GURPS Thaumatology: Chinese Elemental Powers. There are several Detect-based abilities that do not say anything about detecting the nearest subject, implying that Detect works like a normal sense against everything nearby.

Okay, there's gotta be something else. Let's open GURPS Thaumatology: Sorcery.

Detect Magic says nothing that the subject must be the nearest source of magic to the caster. However, let's take a look at Sense Life.

"...you can immediately sense all nearby livings beings, sorted by the direction to each one. The GM will roll against your Per (plus Talent), minus the range penalty to the nearest appropriate being, and inform you if you succeed. ... Any level of success here [follow-up IQ analysis roll] allows you to weed out “false positives” from known living beings..."

So, you detect all subjects, but are penalized by range to the nearest subject, and then you may follow-up with an IQ roll to analyze that one (or multiple?) detected subjects are false positives that shouldn't have been detected, but... whom did you detect and at what distance and penalty then? I admit that my English is not the best, but I don't think this spell's description makes any sense at all! And do you know what's the "best" part about this? I based most of my sorcery (and later wizardry) detection spells on Sense Life. I chose poorly. (Note: The first version of sorcery that appears in Pyramid #3-63 has the Seek Fire spell that does mention the nearest subject!)

But aren't we forgetting an important book? Everything about senses is explained in detail in GURPS Powers: Enhanced Senses, a book I praised a lot in the past. On page 6, it describes the Sensory Hierarchy, saying the following:

"Basic senses, if ranged (see above), also reveal the direction to the nearest significant quantity of a substance or the source of a signal. If not ranged, they allow discrimination of points of contact. Examples: Detect; hearing; touch with the torso or limbs."

Oh, finally something aside from GURPS Psionic Powers and Pyramid #3-63 that agrees with GURPS Basic Set. But wait... hearing? Do you really hear only the nearest thing to you? Anyway, we're here to see how Detect works, not hearing. Let's see the examples. There a many Detect-based abilities that say nothing about detecting only the nearest subject. And many of them function in a way that would make them quite useless if they did detect only the nearest subject. Even if you Ctrl-F "nearest", you get two instances - the "nearest library" in the introduction and the "nearest significant quantity" that I mentioned above. Now that's weird, as I expected this book to clear things up.

So, there are two possibilities:

1) GURPS Basic Set is incorrect.

Detect detects all nearby subjects and not only the nearest one. It operates as a normal sense in that regard - you either roll to see if you spotted every subject or make a single roll, find your margin of success on the SSRT, and see the radius of the area where the subjects were detected. We have a multitude of premade abilities that agree with this approach, and only GURPS Psionic Powers and Pyramid #3-63 disagree with this. Well, the schizophrenic Sense Life partially disagrees too, but it also disagrees with itself. We also have a series of Krommposts, where Sean Punch explains that Detect right out of the box has the ability to exclude unwanted subjects and narrow down the search (such as trying to detect only gold with Detect Metal) and dismisses the claim that this should require Selective Effect, +20%, as it is inappropriate for Detect.

2) GURPS Basic Set is correct.

Thus, Detect detects only the nearest subject. GURPS Psionic Powers (at least in this regard) and Pyramid #3-63 are correct. Examples in GURPS Powers, GURPS Thaumatology: Chinese Elemental Powers, GURPS Thaumayology: Sorcery, and GURPS Powers: Enhanced Senses are incorrect. Kromm also is incorrect.

What do we have to support this? THE GODDAMN BOOK that explicitly says that this is how it works and not some internalized houserules! Oh, and also GURPS Power-Ups 4: Enhancements that also explicitly says that Detect can take Selective Effect, +20% to allow it to exlude subjects and narrow down the search. By the way, this clause was added in PU4, it wasn't present in GURPS Powers. And the book is three years older than the Krommposts that say using Selective Effect makes no sense.

If you take a look at who wrote the books, things will make more sense. GURPS Psionic Powers and the Sorcery article from Pyramid #3-63 were written by Jason "PK" Levine. GURPS Thaumatology: Sorcery also was authored by him, but... it's not his best work. However, it seems that he tried to stay as close to RAW as possible. On the other hand, GURPS Thaumatology: Chinese Elemental Powers and GURPS Powers: Enhanced Senses were written by William H. Stoddard. If you check the forums, you will see that people ignore what the books say and blindly follow Krommposts, and I suspect that some of these internalized houserules made their way into the later books through authors, editors, and playtesters. This is not the only example of this that I know of.

Conclusion

I firmly believe that GURPS Basic Set here is correct. Thus, I have to clarify some things for myself and for others with regards to building Detect-based abilities.

1. Base Detect only finds the nearest subject. If you took unmodified Detect Metal and keep detecting your own armor, you are the only one to blame.

2. To exclude subjects and become able to see the one nearest non-excluded subject, you must add Selective Effect, +20%.

3. Adding Reflexive, +40% does turn Detect into an always-on sense, but it does not include the ability to see everyone - you still detect only the nearest subject. However, you can combine it with Selective Effect, +20% to make it work like vision or some other sense, where you roll to spot every appropriate subject. Since Reflexive essentially includes Reduced Time 1, +20%, this is you using free actions to use your Detect, then excluding found subjects and using Detect again. If you want, you can make a single roll and determine the radius of detection via SSRT.

4. If you want to have an active ability to sense every appropriate subject, use Reflexive, +40%; Selective Effect, +20%; Takes Extra Time 1, -10% (to counteract the aforementioned included Reduced Time), and Reduced Duration, 1/60, -35%. Thus, you take a Concentrate maneuver and detect everything for one second. If you want to activate this ability with a free action, remove Takes Extra Time 1, but add something like Costs Fatigue, 1 FP, -5% or something else that'd turn it into a switchable ability.

5. Consistent writing is difficult with multiple authors.