I did my dissertation in epistemology (theory of knowledge), and one of the big issues in that field is the nature of our basic perception. Is it “pure” or interpretive? On the one hand, you have John Locke saying that the mind is a tabula rasa, a blank slate on which experience writes. In building concepts and propositions, in construing reality, we start with sights and sounds and smells and such. And once we have them in our inventory, we can mix and match them, imagining unicorns and griffins. It's a long story, but the Locke/Berkeley/Hume family of thinkers (British empiricists) provoked a different coalition—Kant/Fichte/Hegel (German idealists)—who said that the mind (or Mind) was very active in ordering experience and that “sense data” were not simply raw material for our sorting.

Perhaps, you’ve seen the apocryphal paragraph (allegedly from Cambridge University) that shows how you can jumble letters and still produce a readable text:

Aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.

The real story is more complex, but the point holds, that we do a lot of unconscious rendering of what appears before our eyes. 

In the same conversation, philosophers home in on optical illusions, such as the hag/young woman, the goblet/faces, and the duck/rabbit. I was used to seeing them in books, but one appeared before my eyes on the back of truck I approached on I-40 between Cookeville and Nashville, as I made my way back from a Humanitas Forum board meeting. I’m pretty sure the trucking company didn’t mean it that way. 

Check it out. At first glance, I saw a bird, something like a puffin (not a duck)—a seabird with a big beak, landing to the left, webbed feet and wings outstretched, scarf trailing in the draft. But that same bird’s eye could be a rabbit’s eye, looking up to the right, behind a bump for a nose. It’s also above and to the left of a dark mark that could be a mouth in something of an “Oh, my!” configuration. And the bird’s beak, upper and lower, could be the rabbit’s ears. Yes, I haven’t forgotten the wings and feet, but focus on the head and it’s weird. 

Or maybe I’m weird for noticing it.