What is "the blues?"

One of the original bluesmen, Son House, answered the question this way:

"This thing that people call “the blues”… The Blues… B.L.U.E.S. Now you take a youngster, when they first wanted to learn to play the blues, they didn’t know yet what the real blues is. Now, it’s according to what you have in your heart. You can have the blues about your fellow man, your fellow lady, or somebody. You can have it from the heart. Now the blues come according to what you mean.

You know, some people can have the blues so hard they go to the river and jump in and drown themselves. Some kill one another, cut each other’s throat and shoot them. You can do anything with the blues. That’s their trouble. They got the blues! They say, “That woman told me she loved me. Now she’s gone. I’m gonna find her and kill her! Something has got to happen.”