Shouldn't we think when sitting?

A layman asked curiously : Does sitting meditation mean one shouldn't think of anything? That's very difficult, isn't it?
Zen Master : Sitting meditation doesn’t mean that one shouldn’t think, but more important is not to chase after or attach to our thinking.

Commentary:

When you are walking, do you attach to the floating cloud? You just continue to walk whether the sky is clear or dark. It is the same, when we are sitting meditation, this constant chain of thoughts is like floating cloud. If you do not attach to your thinking, you can return to your breath easily.