How does one perceive light and shadow?

How does the human eye (in ArM5) perceive light or shadow?

I contend it never perceives light or shadow per se. It perceives lighted or shadowed objects. If there's a lot of light hitting an object, a lot of species will be carried to the eye, providing a sense the object is highly illuminated. If there's little light hitting an object, few species will be carried to the eye, providing a sense the object is in shadow. This has subtle consequences on PeIm spells.

Almost any object under a source of light has a "shadowed" side. Not pitch black, but visibly darker. If you assume that shadows can be perceived directly, then it's quite difficult to make stuff Invisible - a shadowy residue will be left in almost all circumstances. But if you assume that in the absence of species you can't see an object at all, whether it's lighted or shadowed... then an invisible object in shadow stays invisible. This is good.

However, if shadow is "lack of species" (and the fewer the species, the deeper the shadow) this means that if you destroy all the optic species reaching someone's eyes, that person should see perfect blackness; if you destroy all species in a Container (e.g.a Room, or a Circle) every person in that Container should see perfect blackness. I think this does not contradict any published effect, but I may well be wrong.

Look here. Yes, it is complicated.

It's not that complicated.

It just says that there's an additional form of perception that deals with intrinsically luminous objects, like fire, which are perceived by their "luminosity" (lux). But for everything else, which is more or less powerfully illuminated by a greater or lesser abundance of "ambient light" (lumen) and does not give off lux, the argument I was making seems to remain... with the possible exception of blackness if one destroys all species but remains in direct sight of an intrinsically luminous object - in this case someone should see the intrinsically luminous object on a deep black background.

Hmm. I say seems, because the theory is not quite complete as presented in the post, given there's almost no discussion of radiosity. And besides, it's not entirely clear how "canonical" (for Ars Magica) it is. Anyway, thanks for the pointer. I had forgotten about that thread despite having participated in it!