If everything we know comes from our perception of the world, and reality is really just an experience in our minds, optical illusions may suggest what we perceive and what is real are different and can never be the same. Or, failing that, they at least suggest that there is no way to prove that our senses are telling us the truth about the physical world. In philosophy, we call this the argument from illusion. The argument says that if we can imagine a situation in which we can't perceive the difference between what is real and what is illusion, we can't be sure anything we experience isn't an illusion.

These illusory situations are not uncommon. For instance, when you look up at the sky, sometimes the Moon appears larger than the Sun. Because of astronomical science, we know that this is only because the Moon is closer to us, and that the diameter of the Sun is actually more than 400 times the diameter of the Moon [source: NASA]. But if we did not have this knowledge, and all we had to go on was our naked sense of sight, we would probably conclude that the Moon was the greater of the two bodies. If our own senses could deceive us in this basic way, what's to say that they couldn't have been deceiving whoever gathered the data that determined that the Sun was actually the larger object? How can any empirical observation be trusted?

The same principle that gives us this philosophical doubt also enables many of humankind's greatest achievements. All kinds of artists have relied on optical illusions to achieve the effects they desire in their works. Linear perspective, which was first employed by Italian Renaissance architect and painter Filippo Brunelleschi, creates a sensation of depth and distance in a painting. The French style of art known as trompe l'oeil, which translates as "trick the eye," is devoted almost entirely to creating illusory artworks that integrate with their three-dimensional surroundings. For example, a trompe l'oeil painting mounted on a wall might appear to be an actual window within the wall that opens to the outdoors.

Many portraits -- perhaps most notably, Leonardo da Vinci's "Mona Lisa" -- have also been painted in a way that creates the optical illusion that wherever an observer stands, the subject of the painting appears to be looking at the observer. In popular parlance, "The eyes follow you." This is accomplished by playing with elements of light, shadow and perspective.