Thomas Anthony Harris is a person, just like you and me! He's a very smart man who knew a lot about how people interact with each other. He wrote a book called "I'm OK – You're OK" that became very popular, and it helped people understand how they could be good friends with each other.
You know how sometimes you might get mad at your friend if they do something you don't like? Well, Thomas Anthony Harris helped people understand that sometimes it's important to try and see things from their friend's perspective, so they can work things out and be happy together.
Even though Thomas Anthony Harris is not with us anymore, people still read his book and learn from it today. That's pretty cool!