1. Caston concedes that when a contrast is drawn with horasis, then opsis means a faculty. Yet he says that "by itself... [opsis] remains unmarked and can signify either a capacity or activity." He cites Bonitz as an authority but oddly gives impossible Bekker numbers for the relevant text: his note 26 on p. 762 reads "Against Horn 1994, p. 29. See Bonitz (1870) 1955, 553a55-554b7, esp. 553a55-b30." (Bekker page 553 has only 32 lines.)
2. He refers to the first passage from DA that Johansen cites (2.1.412b27-413a2) but seems to suggest that it counts for rather than against his interpretation:
Earlier in the treatise, Aristotle employs an analogy with vision and perception in order to clarify his definition of the soul (2.1, 412b17-413a1): the soul, like sight, is what a natural body capable of life first attains (412a21-28; cf. 2.2, 413b11-13), namely, a capacity for certain activities, in contrast with the activities themselves, like seeing, which constitute its higher attainment.3. Caston points out that aisthesis can mean either a capacity or an actualization, and he maintains that terms for sensation, including opsis, behave in the same way: "The word 'hearing' (a)koh/), [Aristotle] notes, can be understood in both ways, as can all the words used for perceptions an perceptibles (426a7-9)." The passage Caston cites is this, which I'll give in the Hicks translation:
DA 3.2.426a7-9. The actuality of the resonant, then, is sound or resonance, and the actuality of that which can hear is hearing or audition, hearing and sound both having two meanings. The same account may be given of the other senses and their objects.(Note that this passage occurs just before the third passage which I said Johansen could have used as evidence of his view.)
h( me\n ou}n tou~ yofhtikou~ e0ne/rgeia& e0sti yo&foj h2 yo&fhsij, h( de\ tou~ a)koustikou~ a)koh_ h2 a1kousij: ditto_n ga_r h( a)koh&, kai\ ditto_n o( yo&foj. o( d' au)to_j lo&goj kai\ e0pi\ tw~n a1llwn ai0sqh&sewn kai\ ai0sqhtw~n.
Another passage which seems to apply the point exactly to opsis is just a little further below, 426a20-26, which Caston also cites:
DA 3.2.426a20-26. On this point the earlier natural philosophers were in error, when they supposed that without seeing (opsis) there was neither white nor black, and without tasting no flavor. Their statement is in one sense true, in another false. For the terms sensation and sensible thing are ambiguous. When they mean the actual sensation and the actual sensible thing, the statement holds good: when they mean potential sensation and potential sensible, this is not the case. But our predecessors used terms without distinguishing their various meanings. [Again, Hicks.]
a)ll' oi9 pro&teron fusiolo&goi tou~to ou) kalw~j e1legon, ou)qe\n (20) oi0o&menoi ou1te leuko_n ou1te me/lan ei]nai a1neu o1yewj, ou)de\ xumo_n a1neu geu&sewj. th|~ me\n ga_r e1legon o)rqw~j, th|~ d' ou)k o)rqw~j: dixw~j ga_r legome/nhj th~j ai0sqh&sewj kai\ tou~ ai0sqhtou~,tw~n me\n kata_ du&namin tw~n de\ kat' e0ne/rgeian, e0pi\ tou&twn me\n sumbai/nei to_ lexqe/n, e0pi\ de\ tw~n e9te/rwn ou) sumbai/nei. (25) a)ll' e0kei=noi a(plw~j e1legon peri\ tw~n legome/nwn ou)x a(plw~j.