Can a spell detect the invisible?

But in ArM5 we know for sure species are particles (physical entities), right? This is a place where ArM5 and Aristotle would seem to disagree and thus our examination of things would have to diverge at least somewhat from Aristotle's view. Why do we know they are physical entities in ArM5? While they may be weightless, mostly intangible and invisible, and limitless, we still know explicitly that

Species are particles (HoH:S p.61)

We also have spells like False Window (HoH:S p.68) which create magical species that can be blocked by MR. And there is Scattering like Light (HoH:S p.63) that turns something into iconic species that then later turn back into the material they were.

Does it? Last time I checked it could be read as if it's making clear that invisibility is a type of illusionary concealment.
It does make a clear distinction between naturally invisible things and things made invisible, however.

This is whole different discussion, about what the rules should be to provide the best gaming experience. This one is about what they, allow or imply as they currently are.

But I disagree with your statement in several degrees. The spells are not making Second Sight moot. Neither illusions in general, nor invisibility. They are not particularly low level either. To see through invisibility we are generally looking at a lvl 25+ spell, possibly more. And don't forget that both spells under discussion need to penetrate (one of them can tell you there is something invisible w/o penetrating, but sometimes that's not enough).

There are spells that make things obsolete. This is hardly one of them.

I accept it. I just don't try to come up with an unnecessary explanation for the underlying physics when it's not necessary. Magic does what magic does.
I also don't try to come up with an explanation on how a flame created with Ignem burns without a combustible material, for example.
And no, optics does not explain an image w/o substance (and therefore incapable of blocking light), while overlaping another, completely hiding it.

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ArM5 p.67, Second Sight:

You are able to see through illusionary concealment and disguise, including invisibility, and can also see naturally invisible things such as spirits and the boundaries between regio levels.

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Hmmm. You are right.

We appear to have here Roger Bacon's philosophy of nature : optics as the model of natural interaction.

As Lindberg puts it (DMP, Introduction), in this work, Bacon does not formally examine divine illumination of the intellect, universal hylomorphism, the plurality of forms (properly qualified), and the separability of soul. He restricts himself to presenting a full theory of the physics of light. Species is the first effect of any natural agent. As Lindberg puts it (DMP, Introduction), “This is a complete physical and mathematical analysis of the radiation of force--and, thus, of natural causation.”
The use of ‘species’ in this account is not that of Porphyry’s logic or the perceptual notion of likeness. It is“the force or power by which any object acts on its surroundings.” It denotes “al-Kindi’s universal force, radiating from everything in the world to produce effects.” As Bacon himself notes,
species [force, power] is the first effect of an agent…the agent sends forth a species into the matter of the recipient, so that, through the species first produced, it can bring forth out of the potentiality of matter [of the recipient] the complete effect that it intends ([DMS], 6–7). This is a universal theory of natural causation as the background for his philosophy of vision and perception. Most importantly, species is a univocal product of the agent. The first immediate effect of any natural action is definite, specific, and uniform. This production is not the imparting or imposition of an external form. The effect of the species is to bring forth the form out of the active potency of the recipient matter. (DMS, 6–7).


Recent scholarship on Bacon’s philosophy of nature, especially that of Yael Raizman-Kedar (Raizman-Kedar, 2009; Raizman-Kedar, dissertation, University of Haifa, 2009) places great emphasis on the importance of the concept of species for Bacon’s account of perception and mind. She is correct to note that Bacon places a kind of natural activity in the mind via species. This leads in her view to natural species having a direct role in intellectual activity as such. One could speak of the direct presence of material, corporeal species in the intellect itself (see section 5.2 above for Bacon’s complex discussion of the role of signs in animal and human knowing).

Of course, from there is still a long way to HoH:S p.61 species - but at least species as particles become likely. Perhaps the author, @Timothy_Ferguson , can enlighten us about it.

Anyway, once we have ArM5 species as particles moving in a medium like light, Imaginem magic can easier make them pass though invisible objects and enter light again once they are through.

ArM5 p.144 Phantasmal Fire creates light and an illusion of warmth, but does not ignite, burn or melt anything. Magical fires are another thing.

Apparently we disagree here.

We can read up on species with Roger Bacon here:

We find there:

There is also an ontological difference between virtus and species: virtus is a real being, and the capacity for the full realization of a potency; a species is its first effect, having a deficient being. Virtus exists absolutely in the medium; species exist there intentionally. Bacon had a unique understanding of the meaning of the ‘intentional’ existence of the species in medio. While among his contemporaries ‘intentional’ was considered equivalent to ‘spiritual’ (though not necessarily ‘mental’) and opposed to ‘natural’, Bacon thought of ‘intentional’ as having a weak and incomplete being.54 A species, Bacon wrote, in relation to a ‘real’ being is so deficient that it cannot be enumerated among the things of this world. It “is not called a thing, but more the similitude of things”.55


A species does not advance in the medium by locomotion; it regenerates itself in consecutive parts of the medium. The production of a species, Bacon explains, involves a true and natural transmutation of the substance of the patient, which is made by true generation (per veram generacionem). The patient in this case is any receiver of a species, be it the medium or the final recipient.64 A virtus, by contrast, passes through all sorts of mediums without affecting them; it affects only the substances predisposed to receive its influence.

Very much simplified: Bacon's 'species' are not things. But certain things have the 'virtus' to produce 'species' in a kind of Aristotelean "emanation", which pass through an adequate medium and affect a predisposed substance.

That works well to describe sensory species and was likely their inspiration. But also Bacon's species are not particles.

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You clearly misread me, by accident or purpose.

Yes, we disagree on medieval optics being capable to fully explain the inner workings of a fictional magic system created 800 years after the setting.

While a detailed discussion of what species were thought to be by ancient scholars may be interesting, I'm not sure that really helps us. In the real world, they were eventually proved to be incorrect regarding the senses. And in the universe of Ars Magica, where magi would be able to test these things, species may actually work a little differently.

All we really have, that we really rely on, are what the various rule books tell us about how the magic does:

  • The description of Second Sight tells us that invisibility can be either the result of "illusionary concealment" or "natural" (for spirits and regio boundaries). In both cases it can be seen by someone with Second Sight. (ArM5 p.67)
  • Intellego Imaginem has a general guideline to discern illusions. (ArM5 p.145)

To me, it seems that InIm can see through "illusionary concealment", no matter what the exact details of how it is achieved (PeIm destroying the species or a creature with the supernatural ability to become invisible). It probably cannot discern spirits (because they are naturally invisible). It cannot see regio boundaries -- that is covered by Vim.


InIm can detect some spirits - namely those associated with the form of Imaginem.

Intellego (Form) can in general be used to detect creatures with Might associated with (Form)

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The following part of Yael Kedar's explanation of Bacon's species can be read as a medieval abstraction of waves:

A species does not advance in the medium by locomotion; it regenerates itself in consecutive parts of the medium.

Did then around 2006 some quantum mechanicist whisper about wave-particle duality into @Timothy_Ferguson's ear, thus leading to

Species are particles (HoH:S p.61)

? :thinking:

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28 posts were split to a new topic: Alternative Methods of Invisibility

I see... so basically, anything one does with Cr/Mu/Re/Pe Imaginem gets the light wrong. You create the illusion of a big castle? You still get blinded by the sun while resting in its shade. You make someone who's very fat look very thin? The shadow is still fat. You make someone's (heavy) clothes invisible? He won't look naked, he'll look ...matte black, like a ninja. Wizard Sidestep is almost useless in daylight against someone who know's what's going on.

Frankly, this is problematic. I don't think it's at all obvious to the average reader of the corebook.
And I've not even given significant thought to what happens with echoic or olfactive illusions.

It's really a bigger can of worms than Magic Resistance. I'm tempted to say that the easiest fix is that Imaginem works as Phantasmal Force in that other game, as illusions in most legends, so it gets the light right. Mechanically, it would just require Imaginem to be able to manipulate light "entangled" with species, to be able to remove an object's interaction with it, and to create an illusion of light which illuminates just as the real thing. Does not seem like a big deal, and way easier than the frustrating complicated thing this is devolving to.

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Or just use Ignem requisites (and possibly a Technique) when you want something be an illusion and also have a shadow or similar?

It may be problematic, but light is clearly Ignem and species are clearly Imaginem. The notes about the invisibility spells help clarify that point if it's missed.

As far as illusions in general go, don't forget that glamours are essentially real illusions, and being real in that sense they will block light if the object they imitate would. So it's not like all illusions need to work this way. Just Hermetic ones when someone doesn't want to involve Ignem.

This seems like it to me.

And to address the second question in your original post:

Not sure to what extent it was discussed in this thread, but as I understand this should rely on the standard mechanism of your senses. Ordinarily your vision depends on species, leaving invisible objects out of the scope of this guideline.

InIm 15
R: Per, D: Sun, T: Vision

Allows the caster's vision to compare what it sees with what the caster would feel if he touched the images. This has two main effects. The first is that visual illusions that don't overlap with physical objects or that can't otherwise be touched are transparent, seeming hollow like smoke. A fake wall or person is easily identified as such provided the Imaginem effect didn't cover the touch sense, but a Per+Awa check may be needed to spot details such as a person's illusory hat. Cosmetic illusions that closely match the target it covers are typically not detected, however. The second is that invisible characters and objects which are otherwise tangible will appear as a sort of outline, allowing the character to discern their rough shapes. In combat, this is enough to make an invisible enemy obvious (see HoH:S 33). This spell leaves no outline for creatures and targets that are both invisible and intangible.

(Base 1, +2 Sun, +4 Vision)

This would need to penetrate, since if the spell if unable to register a sense of magical touch then it isn't possible to compare with the visual input.


That was the goal. But light is definitely Ignem. And magi should be able to see illusions that don't Penetrate.

The mess is the result of trying to produce something coherent and medieval that gets those effects. I think it's actually really close, but I don't think it quite works. I suspect that the target set of effects is not logically coherent.

What about InVi, Target vision - a spell which outline magic (spells or persons). maybe with a CrIm effect to let it shine for everyone - aka Farie Fire?

If you force-less cast, it won't affect magi, creatures with Might, or anything with MR. Maybe you'll catch an invisible grog, but that's it.

The unpleasant part about the original spell is that it actually detects MR without any effort.

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