One example of something that you can see but doesn't exist is an optical illusion. Optical illusions are visual phenomena that trick our perception and make us see things that are not actually present or distort the appearance of objects. They occur due to the complex ways our brains process and interpret visual information.
For instance, the famous "impossible objects" such as the Penrose triangle or the Escher's Waterfall appear to be three-dimensional objects, but they cannot exist in reality because they violate the principles of geometry or gravity. Similarly, illusions like the Müller-Lyer illusion, where two lines with arrows pointing inwards or outwards appear to be different lengths, despite being the same, showcase how our visual system can be deceived.
These optical illusions highlight the intricate nature of human perception and demonstrate that what we see is not always an accurate representation of reality. They serve as intriguing examples of how our brains interpret visual stimuli and can create perceptual experiences that differ from the objective truth.