Just add a magnitude. If you can destroy the image of a person, surely you can destroy the image of his shadow without actually playing with the stuff of the shadow itself.
If you think that makes the spell too easy for your saga, add a magnitude or two to the base.
Perhaps 'PeIm invisibility not only destroys the species emanating from an object, but also destroys the object's ability to block other species'.
That roughly matches the 'overwrites' suggestion without being quite as metaphysically weird, but does still leave the 'you can see a shadowy shape if the invisible thing is sufficiently backlit' problem.
A shadow is not an image in the Imaginem sense. For instance, a square with the shadow of a circle cast over the middle of it is just the image of a square in somewhat eccentric lighting, not the image of a square with a circle in the middle (bolded for clarification purposes, not to convey some sort of angry tone.) Similarly, that's why creating a believable image of a torch is CrIm(Ig) -- because otherwise the torch-image would be no brighter (or hotter, if it mattered) than the ambient light.
My understanding is that Imaginem does not include the following things we might otherwise think of as sensations: 'light', 'darkness', 'heat', 'cold', 'pain', and various internal or emotional 'feelings'.
I think your example is blending the pseudo-Aristotelian species theory of AM5 with actual optics. My knowledge of Aristotle's theory of intromission is not the greatest, but I don't understand how it explains shadow.
The Aristotelian theory is of course out of period for the game anyway. Alhazen had written extensively on more modern optics two centuries before, his book was available in Latin, and Roger Bacon was expanding on it in the West not much later than the canonical game start. Even so, I appreciate using Aristotle in game because it provides a way to explain IM as a discrete form, rather than using IG for light illusions or ME for mental illusions. When, however, it's taken so literally that you can't create an image of a torch without an IG requisite or do invisibility without both RE and PE (and apparement IG for the shadow), it gets too complicated for my taste in gaming. If I want my game to be a physics experiement I'll at least use real physics.
Could be. I don't know Aristotle well enough to be sure, so my conception of how Mythic Europe works usually just follows from 'how can I be relatively consistent with the setting and metaphysics that have been described to me?'
So, taking as facts:
Perdo Imaginem makes things invisible.
Perdo Imaginem invisibility still casts a shadow
Creo Imaginem does not create light
I have extrapolated that 1) images created by Creo Imaginem must still be lit by the ambient light, 2) objects made invisible by Perdo Imaginem allow species to pass through them, else they would just be dark blobs rather than invisibile, and 3) objects made invisible by Perdo Imaginem don't actually allow light to pass through them as such, but if they're not extremely backlit or something the result is somehow sufficient to fool the eye.
I don't understand how the intensity of background light affects perception of species. With respect to shadows, I don't understand how that fits in with a shadow, being caused by the body blocking the sun's rays in one direction causes the darker "shadowy" perception but does not fully block perception. Maybe we just don't understand Aristotle very well or maybe these sort of questions are why his theory was abandoned prior to the AM period.
Working with the theory is a fun thought experiment and I have to admit some of the consequences - torches that look real but don't help you see for example - sound interesting but to me it complicates the game too much.
So as I understand it:
We observe that with PeIm magic an object can be made invisible, which also allows us to see what's behind the object; but making it invisible doesn't stop the object from casting shadows or blocking light from light sources (not without additional magic at least).
We conclude that PeIm destroys both the species emanating from the object and the object's ability to block other species, but not the object's ability to block light. Seems fine to me.
Does this mean that we could add an Ignem requisite (and probably a magnitude as well) to further destroy the object's ability to block light, thus eliminating those pesky shadows?
I know there are other ways to get rid of the shadow, but I haven't seen the Ignem-requisite possibility suggested.
I think there's some confusion between how species work and how light works. My recollection is that species are emitted from the eye and move out until they hit a visible object. At that point the object is either instantaneously perceived, or the species are reflected back to the eye and perceived that way. This would mean then that a PeIm spell destroys the image, thus allowing species to pass through, but the species themselves are not affected.
This is how my first SG described it to me way back with ArM2. ArM5 may be different though. I seem to recall that the species are in fact the target of such spells now. This may offer some insight into why invisibility works the way it does though. It's an artifact of a previous paradigm.
That was indeed a medieval view of how things worked. It was not the Aristotelian view, where the object itself altered the medium around it, in some sense "broadcasting" species.
This is actually a pretty interesting observation.
We are in 1220. That view was proven wrong 2 centuries ago by Alhazen. His Book of Optics has just been translated into Latin. Magi had an easy way of testing both theory, so the truth has been known for longer. But who knows if we share the same truth as Mythic Europe.
That's the extramission theory of optics, and is not current in 1220. An alternative to extramission was intromission, where the air was 'stamped' or 'imprinted' with colours or shapes that struck the eye; or else there was a flux of particles which came from the object to the eye. Aristotle rejected boith intromission and extramission. His argument against them was based in part on the reasoning that under these theories, you should always be able to see as long as your eyes are open. However, in a dark room this is clearly not true. Instead, he proposed that light caused the emanation of species which were then transmitted through some transparant medium (e.g. air or water), which is The Truth in ArM5.
Euclid and Ptolemy championed extramission, but Al-Kindi discovered errors, and Avicenna popularised his reasoning. These scholars also linked Galen's anatomical works to optical theory, explaining how the hollow nerves transmit species to the soul. The species argument is the most commonly accepted version amongst both Christian and Muslim intelligensia.
(NB: probably an oversimplification of the situation, but I don't have sources on me).
