There are many things in life that some people might see one way while others see it a different way. Depending on how you perceive it, you can view a large range of things without actually seeing them. "To perceive is to create a figure or shape that does not necessarily appear as such in the real world but that we can represent mentally so that we can recognize it under various conditions." Usually optical illusions effect the cortex, but some can also effect the retina. In the following picture, some may see a duck while some may see a rabbit. Depending on how you perceive it, you may only be able to see one of the animals, or maybe even both.