There are several more of a similar nature.  I follow the “if you only write when you’re inspired, you write as a hobby, not a lifestyle” camp.  Don’t get me wrong, it’s wonderful when inspiration happens, but if I only wrote when I was inspired, I would get a lot less writing done.  Plus, you can always force your brain to come up with ideas.  Some people say it’s impossible simply because making your brain form new ideas can be really hard the first couple of times.  Once you get the hang of it, though, it becomes easy to grab inspiration instead of having it come to you.