Quotations are mainly used for what

It's challenging because it demonstrates the principle of relative height. Objects in our upper field of vision appear farther away than objects in our lower field of vision. Vertical objects appear taller than objects near the ground, which are closer to us. The Ebbinghaus illusion demonstrates the difficulty we have perceiving objects in contrast with their surroundings. There is also the Poggendorf illusion that makes two lines appear to be joined when they are not. There are many more illusion tests that makes something seem like it's to our profound knowledge.