I'm confused again. Publicly expressing confusion seems to have become a (perhaps tiresome) professional habit of mine these days.
A classic example of an illusion -- classic in the sense of dating back to ancient Greece, not classic in the sense of central to 20th century perceptual psychology -- is the oar partly submerged in water. Due to (what we now know as) the laws of the refraction of light, the oar typically, in some sense, "looks bent" as it angles into the water. But one might argue (does John Austin argue this?) that that bent appearance is not really an illusion: If one knows enough about the world, one should know that an oar partly submerged in water (seen from a particular viewing angle) should look bent just like that. If it looked straight, I suppose, a longtime oarsman or a person very familiar with the laws of refraction might think the oar looked strange, might even think that it looked like an oar that is actually bent (bent in such a way as to exactly compensate for the bend a straight oar would seem to have at that angle). Perhaps part of what it is for an oar to look straight is for it to also (in a different sense) "look bent" when it is partly submerged in water. (This formulation is indebted to Alva Noe's "dual aspect" view of perspectival appearance.)
So is the skilled oarsman experiencing a visual illusion as he looks at the oar? If we say no, then I'm worried we're off onto a slippery slope to entirely denying the possibility of illusions that are known to be such. When I press gently on the side of one eye, I seem to see double. Is this also no illusion, since I know that's how things are supposed to look when I press on one eye? When I look at the Poggendorff illusion (below) I know that if the upper segment of the line going behind the rectangle really is aligned with the lower segment emerging near the bottom, they should look (in some sense of "look") offset. I've seen this illusion so much that now I think that if I saw a figure of this sort where the lines didn't look (in the relevant sense) offset I would probably infer that they really were offset. Parallel to the oar case, perhaps, what it is for the line segments to look aligned (for me) is for them to look (in a different sense) like they don't align.
(image reproduced from Titchener 1901-1905)But if the Poggendorff illusion is no illusion for me, then is any illusion an illusion to someone who knows how the illusion works?
So maybe we should say the oarsman does experience illusion. But then there's the risk of a slippery slope on the other side, toward saying that much more is illusory than we ordinarily think. If the bent oar is illusory, it seems that looking at things through a curved glass of water must be, since the refraction similarly distorts things. And then it seems like a magnifying glass held at arm's length similarly creates illusion. But then also does a magnifying glass held near the eye? A telescope? Ordinary corrective lenses?