In problem 1, it is easy to find the answer. if X/2=3, then X=3*2, which means X=6. this equation has two known quantities, and one unknown. Piece of cake! but in problem 2, we have two unknown quantities, X and Y. Without knowing the value of Y, we cannot determine the value of X. We might say that there is no unique value of X. And without knowing the value of X, we cannot determine the value of Y. So there is no unique value of Y either.

By now, you may be wondering what these problems have to do with Depth Valley. The answer is simple. When we look around the world, our eye must solve problem 2. We need to determine how far away and how large things are. So there are two unknowns: distance and size. And we have only one known quantity: the apparent or visual size.