A 'Default' Meaning for OPSIS?: Texts (Once Again)

I want to raise the question of what opsis (o1yij, 'vision') means at DA 3.2 425b12-17. This is crucial for deciding between the 'activity' and 'capacity' interpretations of the passage.

Johansen claimed in his BACAP seminar (i) that the 'default meaning' for the term in DA should be the faculty or capacity of sight, (ii) that the term can intelligently be read at 425b12-17 as having that sense, and therefore (iii) that the term should be read as having that sense (the 'capacity' interpretation').

Caston maintains, as against (i), that the term in DA can just as well mean an act of seeing; as against (ii) that it cannot intelligently be read at 425b12-17 as meaning anything other than an act of seeing; and as against (iii) that the term therefore should be read in that way (the 'activity' interpretation).

So here we have a good, old-fashioned, head-to-head dispute. Who is right? On each point, I'll give the arguments for both sides: readers of Dissoi Blogoi should then feel free to comment or render judgment. I'll also render my own judgment (hoping thereby to provoke dissent!).

First we need to consider (i), whether opsis has a 'default meaning' in DA, as Johansen claims, looking at the arguments on both sides. Here is the passage, once again, for ease of reference:

GREEK TEXT
)Epei\ d' ai0sqano&meqa o3ti o(rw~men kai\ a)kou&omen, a)na&gkh

h2 th~ o1yei ai0sqa&nesqai o3ti o(ra~, h2 e9te/ra. a)ll' h( au)th_ e1stai

th~j o1yewj kai\ tou~ u(pokeime/nou xrw&matoj, w3ste h2 du&o tou~

au)tou~ e1sontai h2 au)th_ au(th~j. e1ti d' ei0 kai\ e9te/ra ei1h h( th~j (15)

o1yewj ai1sqhsij, h2 ei0j a1peiron ei]sin h2 au)th& tij e1stai au(th~j:

w3st' e0pi\ th~j prw&thj tou~to poihte/on.

JOHANSEN'S TRANSLATION
Since we perceive that we see and hear, it is necessary [b12] that one perceives that one sees either by sight (opsis) or by some other [sense]. But the same [sense] will be [b13] of sight and the underlying colour, so that either there will be two [senses] [b14] of the same thing or it [the sense] will be of itself. And furthermore, if indeed the sense of sight were different [b15], then either it will go on to infinity or some sense will be of itself, [b16] so that we should do this in the case of the first [sense].

CASTON'S TRANSLATION
Since we perceive that we see and hear, it is necessary either by means of the seeing that one perceives that one sees or by another [perception]. But the same [perception] will be both of the seeing and of the colour that underlies it, with the result that either two [perceptions] will be of the same thing, or it [sc. the perception] will be of itself. Further, if the perception of seeing is a different [perception], either this will proceed to infinity or some [perception] will be of itself; so that we ought to posit this in the first instance.