Do you believe in Writer's Block?

"When I sit down in order to write, sometimes it’s there; sometimes it’s not. But that doesn’t bother me anymore. I tell my students there is such a thing as “writer’s block,” and they should respect it. You shouldn’t write through it. It’s blocked because it ought to be blocked, because you haven’t got it right now." --TONI MORRISON

I know that some people, unlike Morrison, don't believe in writer's block. What do you say? There are definitely times when I sit down to write and it just doesn't come. Is that writer's block? Or is it that the ideas and direction I'm heading aren't yet clear to me.

Stephen Spender says that, "The best thing is to write anything, anything at all that comes into your head, until gradually there is a calm and creative day."