Yup. And the Supernatural Abilities paragraph or the specific Abilities description gives guidelines/inspiration for this.
I disagree with you Ben - for two reasons:
Even if this is about sensing something I think you are making a mistake by making this about species. The effect that makes someone appear different or not to be seen altogether is not some effect put on the species, but an effect on the object "emanating" those species and thereby altering the species by proxy. Enchanting, with whatever powers or Hermetic spells arent to be found in the core RAW (If I recall it is mentioned as an option in one of the House-books). The rules on species have sparked several debates - most of the ones I recall from this forum often concluded with an agreement that those rules arent entirely simple, but the alternatives would cause more problems than ones at hand (basically the concept of non-resisted species are needed - but they shouldnt on the other hand become a loophole to avoid MR the other way around).
No matter the triviality of the information gained from a Sense-based spell, that information is dependent on penetration, because even if trivial the information is magical. The point you refer to, that the mundane sense isnt resisted, is needed for many other reasons. It means I'll be able to see a magus even if his MR stops cold my spell effect (e.g. a Vision-based detection of vis). If my general eyesight would suddenly be resisted it wouldnt only handicap me immensely, cause various hillarious scenes, it would also make for an useful abuse as any low level Sense-based Intellego spell, regardless of form, could be used to spot people, things, or creatures with MR (or rather: not spot).
Sure I can agree that this is not like the ReIm effects that alter the source of species or make those species appear emanate from a different location... but I'm suggesting that the ghosts do emanate a species, simply one that mundane people lack the sense to detect.
I think there's a word missing from that first sentence-- probably species?
I can certainly agree that enchanting species would be a level of detail that could get quite messy very quickly. I don't think that's where I would want this discussion to go.
From the other direction, though, I'm trying to provide an ability similar to Second Sight without requiring a magus to be a diviner or intelligo specialist. I think that needing upwards of 10 to 30 penetration on an InVi 30 spell is something that's going to be beyond the capabilities of most magi (who don't focus in In, Vi, or ghosts?) for a long time (certainly a few years, maybe ten-ish?), and I think that's excessive. I realize what the third sentence prevents, and I'm quite happy it's there to prevent the inadvertant sensory deprivation that might result otherwise.
Really, for my suggestion to be permissible, one has to accept that there are species being emitted by the ghosts, but that those without SS cannot normally observe those species. Given that posit, the penetration doesn't become a requirement as you're no longer affecting the spirit, but rather augmenting your own sensory input to accept what is already present. Even without penetration, InVi 30 is reaching the ceiling of effects possible for a younger magus.
I think we can both agree that the exact nature of ghosts and their (possible lack of) ghostly species is not defined within the rules at all, and so we arrive back at a case of YMMV--
If ghosts simply have no species, then Second Sight becomes much better than a simple minor virtue. It provides In(Im/Me/Vi) of unlimited penetration and has no complications of maintaining concentration. In truth, I think it becomes something just a bit short of Mythic Blood. For this case, yes, even simply perceiving ghosts with In(Me/Vi) would definitely require penetration.
If ghosts have a species that ordinary sight does not detect, but Second Sight does, then it brings SS to back into line of a minor virtue, which, in my opinion, is where it belongs. For this case, no, penetration would not be necessary for simply seeing ghosts with In(Me/Vi) spells.
Ben, you make a fine case with your argument. What I might have missed before was that you were speaking of ghosts in particular. SS covers all kinds of creatures or Hermetic effects that change appearance or disappears altogether - letting Intellego Vision-based spells emulate that without penetration was to me a large undermining of the MR rules. Making a special case of ghosts is another thing, and I missed that part, but I do find the arguments for it sensible - even if I'm not sure I'd use that interpretation in my own saga (would it not be as easy just to make ghosts in ones saga have a low enough Might that penetration wouldn't matter much anyway?).
You certainly could...and that might be a more palatable route for Lebo's troupe.
For myself, I think a swarm of low might ghosts might just be an annoyance, rather than a challenge. It would depend on their role within the story, I suppose. Thirty ghosts with a might of 3 to 5?...worth considering, again I think it depends on what exactly you want to do with those ghosts.
For us, I don't know that the option I suggest would impact the saga overmuch, and I'll likely recommend it to our SG.