The question arose in a different thread, but I think it is worth discussing in more detail:
If a character implants specific cyber- or bioware, that gives him additional senses then the question arises, which of those can be counted as "seing".
Example 1:
The character can hear the guard in total darkness in front of him, but he cannot see him. Sound waves created by the steps of the guard travel towards him, hit his eardrum, and as a consequence he hears the guard. He can shoot in this direction, but he cannot cast a direct spell on him.
If we add spatial recognizer the principle of hearing stays the same but he can already shoot the guard with decent probability to hit him, but he can still not cast a spell.
But what if he has ultrasound now? O.k. he emits sound waves, that hit the guard, are reflected back and from there on it will be the very same as before, i.e. he hears the guard. Nevertheless Ultrasound usually is being counted as seing. That is fine for ranged combat.
But the question is: Does ultrasound constitute seing - in the sense of seing something to make the percept an eligible target for a direct combat spell?
Obviously the information enters the system via the ear. Hair cells sends information to the corresponding areas in the brain, to the cochlear nucleus, superior olivary complex, inferior colliculi, medial geniculate nucleus of the thalamus and finally primary auditory cortex. Right? Or will the ultrasound system be connected to the visual cortex? Probably, as the percept is described in visual terms. But does this mean he is seing? Is information in the visual cortex sufficient for a visual percept?
Example 2:
The character cannot see the guard, because it is to dark. So he cannot cast a spell on him. In seing usually photons hit his retina then the information is send to lateral geniculate nucleus of the thalamus and to the primary visual cortex.
Now we add infrared vision to the system: The character now becomes sensitive to a different spectrum of photons (still electromagnetic waves), and the processing stays just the same. We would say the character sees the guard, he can cast a spell on him.
Now it gets even more complicated when we add radar "vision", or the electromagnetic sense. As vision itself they are is based on electromagnetic waves or fields (just of a different wavelength), but there is neither a natural receiver nor a corresponding system in the brain that usually deals with radar and is used to interpret these signals. Thus: where will it be interpreted in the brain? Probably preprocessed by the implant and then send to visual cortex probably, but we don't know for sure.
In addition, the cortex couldn't process several different inputs at the same time, so this seems to suggest, that several senses are processed in different brain areas (but where?).
So far it seems, that information in the visual cortex seems sufficient to constitute seeing (this is plausible, because the percept would be "as seen" - somehow similar to an visual impression)
But now lets put one problem on top: Spirits see, yes, but different (as they cannot read computer screens, right?) But how? They neither posses real eyes, nor a visual cortex, nor any brain at all. They can target spells.
Or critters, that do not even possess vision. Can they target their powers, yes, they can. What is vision-like in them?
To conclude:
1. What constitues seeing then? Under which conditions can we speak of "seing" in the sense, that it allows us to target a spell with it?
2. If information in the visual cortex/pathway is sufficient to define seing? Why don't we then just add a synesthetic-implant to "see sound"? The technology obviously exists (e.g. within the ultrasound system). That would be coarse, but better than nothing.