The third case study of how we understand the world, how we make an inference about a three dimensional world from a two dimensional array, is a case of depth. So you don't see all of these figures as lying along the same plane. You see, for instance, the man in front of the house. And it's an interesting question, how you do this. What cues your mind uses to figure out the relative locations of things in space. For more dramatic illustration, you could look at this and this is still, it's a two dimensional figure. Just on a two dimension array, but it doesn't look two dimensional. Things are happening to it that give you the illusion of a three dimensional thing. So what's going? Well, there are different answers. One answer is the size of things. So this cartoon illustrates that, if you see something that really takes the size of your thumb, and it's a person, most likely it's not a tiny person floating in the air in front of you. Rather, it's a person at a distance. And so, knowing the rough size of things can help you infer how far away they are. Then, there's interposition. So go back to this picture. The man is in front of the house because the man interrupts the house itself. In another words, you can fully see the man, but the house is blocked by the man. And so, duh, the man is in front of the house. And as I walk along the stage, and walk back and forth, I change in size, but you know I'm not actually changing in size. So you infer that I'm moving away from you or close to you. And as I walk in front of something, you don't assume that my body adopts some weird shape, or I'm half object, half person. But rather you assume that I am moving behind that object. Then, there's other more subtle cues that psychologists have studied. So consider these two illusions. This is the Mueller Lyer illusion. And what's cool is, if you're like most people, you will see the line on top as longer than the line on the bottom, even though they aren't. It's an illusion created by the context in which the lines are shown. And so, what can we say about this? Well, the standard explanation, here, is that the top horizontal line for the Müller Lyer illusion looks longer because we infer that it's further away. The things on the side lead us to infer that it's from a distance because it seems to be pushed away from us. And so, because it's a distance, it occupies x space on our retina, but we assume since it's further away it must really be bigger than x. And so, in our minds, just as we adjust to the colors of objects in shadow, we adjust our perception to size of objects due to distance cues. Similarly, the inward pointing line looks closer. And if something seems to be closer, we think it's smaller. Relative to the same amount of retinal coverage for something that we think is bigger. Or is another case, the Ponzo illusion is the same thing. Because of the converging lines, you get the impression that the line on top is further away. And since it actually takes the same amount of space on your eye as the line on the bottom, you then infer that it must be bigger. And none of this is conscious. This goes back to the rules of language. We're talking about all these subtle rules of language. Where you understand the pig is eager to eat is different from the pig is easy to eat. And you understand it immediately. But you can't articulate why. And the same with the cues. Immediately parts of your understanding of scenes guide you to think that the top line is longer than the bottom line, but you can't say why. And this is why psychology is often so interesting. And that's why the study of vision is in some regards similar to study of language. In that, they both explore mental mechanisms that work with extraordinary speed, unconsciously, to hear some understanding of the world. And I will end with another example on that. In fact, one of my favorite examples. These are the Shepard tables developed by Roger Shepard. These look like different tables. And if you just show these to people, and you ask them something like, which would be easier to get through a door if you have a thin door? People would say the one on the left, they look like very different tables. But this is an illusion set up by cues of depth perception and I'm going to show you this is an illusion in this way, if you think I'm doing some trickery. Rectangle here, [SOUND] covering the table on the left, and now let's just move it. [SOUND] And as you can see, they match. And based on what you know about depth perception, thinking about the Muller Lyer illusions and the Ponzo illusions. Thinking about the cues to depth, I'll leave it as a take home exercise to explain why that Shepherds tables work so well. And that is where I am going to end our discussion of perception, through examples of these unconscious and lightning fast cues, beliefs about how the world works that can create illusions in the context of a psychology experiment. But in real life allow us to understand the physical world so well.