Dolph Sharp

If I kiss you, that is a psychological interaction. - On the other hand, if I hit you over the head with a brick, that is also a psychological interaction. - The difference is that one is friendly and the other is not so friendly. - The crucial point is if you can tell which is which.

It is generally agreed that "Hello" is an appropriate greeting because if you entered a room and said "Goodbye," it could confuse a lot of people.