This one is simple. How would you know what anything without making some comparison to something else? You wouldn’t. You need the opposite of something, because the opposites define each other. If this weren’t the case, the law of non-contradiction would be no good. It would be non-existent, but you can’t have existence without having something to compare non-existence to.
I know what lemon tastes like does this mean I need to know what watermelon tastes like?
The ball is blue. The ball is green. Two contradictory statements but not opposites. I open my eyes and see that the ball is green. Knowing it is green means I have to know what blue is?
Can you compare non-existence?
You need to know what something else tastes like, but it doesn’t have to be watermelon.
Sure, you know what a tree is, which I assume you agree is the contradictory of "ball. You also know that blue is the contradictory of green. That’s how you can tell things apart, unless you want to say they’re all the same, which means I have to wonder how you tell the difference between a tree and a ball, or blue and green.
It seems so. Compare yourself to your great-great-great grandparents. What’s the difference between the two of you? One of you exists and the other isn’t?