As a warm-up exercise, my players and I today started talking about ... interesting uses of illumination with species, as species are one of the /big/ things that make our brains...um... break... with regards to the paradigm. (Though I've failed enough SAN checks as the GM to be quite content with them, now... :: )
Consider the following.
Create illumination equivalent to the light of a candle right in front of one's eyes. None of the light escapes.
(I'm waiving the bases and whatnot, cause it doesn't matter for this discussion)
What would the effect of this light be? Do things generate visual species at all times? If they don't, why don't they? Does this spell provide incorrect illumination of species? Why?
Doesn't light cause the species to be shed? In this case, since the light never reaches the objects, none are shed - the spell will illuminate nothing, as the light is prevented from reaching any object. You're not illuminating anything. Incidentally, it's the same result as in our real world - light that doesn't reach any object won't do you any good.
Now, you would see the light itself. Supposedly. I don't know how they treated that.
Well, my philosophy is that objects exude visual species just as they exude tactile and olfactory species. Audible species are a special case due to unique organs designed for their generative properties. (mmm, special cases.)
Given that species are shed by objects, what role does illumination play?
Illumination is what causes objects to shed visual species. As such, in the dark, no species and nothing to see. This means, for instance, that CrIm can create a perfectly visible illusion in pitch darkness.
Why do cats need to see by visual species? Second Sight doesn't deal in species, for instance. Or perhaps the well known fact that a cats eyes glow in the dark means they emit enough light for themselves to then see by. Bright light must cause things to emit more species, so perhaps the very pale light from their eyes is enough because they've got sensitive eyes.
This question seems to be coming from a moden perspective rather than a medieval one. We today need to know that illumination is made of radiation which stimulates electrons which emit more radiation which comes at our eyes (and stimulates chemical receptors etc etc) to understand the mechanism. But in the medieval paradigm, maybe YR7's sentence is the definition of illumination: the particular property of fire that enhances the shedding of visual species.