Is what we see in this world actually real?
What might be viewed as "real" to one person might not be real to another. For example: Let's say you're looking at a dog. What color is it? Does it have a smell? How does it feel? How does it sound? And most importantly, how do you feel about it? Now ask someone else about the dog. Chances are, you get somewhere between a slight difference in description to a major difference.
Why? What we encounter in this world, like the dog, is broken down and then reconstructed inside our brains, but before it is reconstructed, it has to pass through "filters" inside our brain. These filters are our defects in the senses (like color-blindness, ect...,) defects in the brain (like a chemical imbalance,) our cultural and religious beliefs and prejudices, and our emotions.
So you may see a brown dog. Your neighbor, being color-blind, may see a grey dog. To a chemically imbalanced brain, there's no telling what it might make the dog out to be. You may think the dog is unclean because of your religion, your daughter, not being the same religion, may have neutral feelings toward it. And, due to your bad day and bad mood, you may want to avoid or hurt the dog, while the person your with may fall in love with it and want to take it home.
So anyway, we do agree that a dog is there, though. So we go further into our minds and figure out what happens to the dog once it passes through our filters. You see, our minds are always trying to simplify things for us, so what we encounter in the world is compared to symbols in our minds. No matter if it's a miniature pincher or a giant greyhound, it's considered a dog. So it's linked to the dog symbol.
Our minds are very big on symbols. Ask someone how many sides a pyramid has and they'll probably tell you three, until they sit down and actually think about the three dimensional, five-sided pyramid, rather than the symbol they have associated it with, the triangle.
So is a dog really a dog? Is a tree really a tree? Do your socks actually match? Made you look, didn't I?